Well, start by posting the exact make/model of the ISP router/modem AND a copy of your current Mikrotik configuration, follow the instructions here:
viewtopic.php?t=203686#p1051720

# 1970-01-02 00:05:24 by RouterOS 7.12.2
# software id = mumbojumbo
#
# model = CRS310-8G+2S+
# serial number = mumbojumbo
/interface wireless security-profiles
set [ find default=yes ] supplicant-identity=MikroTik
/ip hotspot profile
set [ find default=yes ] html-directory=hotspot
/ip dhcp-client
add interface=ether1
#error exporting "/ip/ssh" (timeout)
/system note
set show-at-login=no
/system routerboard settings
set boot-os=router-os

What you report is an error at a pretty much "basic" level, if it is something connected to the specific ISP modem/router model it is probable that it already happened to someone else, and maybe a solution already exists.

The fact that it works when you put an old "dumb" switch on the connection points to some kind of timing problem, but it could be also something connected to MAC addresses or *whatever*.

Is the modem/router in router mode or in bridge mode?
I.e. the IP address that the DHCP server on it is serving is a public or a private IP?
If the former is it a static one?
I mean is it possible to try with a static address on the Mikrotik port connected to it?

Assuming your ISP connection is 400Mbps or less ???

Not Working Configuration:

1. ISP box -> MikroTik -> PC

Working Configurations

To my surprise, when I connect through the following means, everything works (LEDs light up, and the connectivity exists):

2. ISP box -> PC
3. ISP box -> ASUS router -> PC
4. ISP box -> Some old AP/Switch -> Mikrotik -> PC
5. ISP box -> ASUS router -> MikroTik -> PC

This is not an issue with the cables (as they all work with different configurations) but rather with the MikroTik.

Do you know how to get deeper into debugging and fixing this issue?

Hi Kaszanas

I had simillar issues..
2 Modems, one did well with the CRS310, the other did not at all (port down).

PC's and other Mikrotik switches worked instantly with both modems.

Drove me insane, until i gave up for a day or so.
Now suddently both modem work like a charm, however, switching the second modem, that didnt worked right away to any other port makes that port to stay down, not seeing a connection too. Putting the modem back to the port it woked once, instantly lights up the port... kinda like the port and the modem needed some time to get in friendship thogether ^^

I have absolutely no clue whats going on there, but now its working as one would expect it (aslong as i dont change ports)


So my advice for you by now:
Let the modem pluged in for 2 days and then check back....

• cables can not be ruled out with excuse of "they work with other devices" ... physical layout of connectors and sockets has some tolerances and even if they are manufactured within allowed tolerances they still could fail to work correctly (i.e. pins in device socket may not create proper connection with connector pins). It's enough that single wire/pin doesn't make proper connection and the whole ethernet connection fails (or falls back to 100Mbps)
• switch and ISP modem can't negotiate a common speed/duplex configuration ... check switch settings ... on ethernet port, and verify that autonegotiation is enabled and all speeds / duplex variants are enabled

Some random ideas:

• cables can not be ruled out with excuse of "they work with other devices" ... physical layout of connectors and sockets has some tolerances and even if they are manufactured within allowed tolerances they still could fail to work correctly (i.e. pins in device socket may not create proper connection with connector pins). It's enough that single wire/pin doesn't make proper connection and the whole ethernet connection fails (or falls back to 100Mbps)
• switch and ISP modem can't negotiate a common speed/duplex configuration ... check switch settings ... on ethernet port, and verify that autonegotiation is enabled and all speeds / duplex variants are enabled

Your at the mercy of the isp modem. Since you don't have access to it.

When we talk about mac address. We're talking about the isp modem registering the mac address of the router connected to it.

What I would do it first.

Unplug the isp modem. Connect the cable to port 1 on the isp modem to port 1 on the mikrotik router. Connect a pc to port 2 on the mikrotik router.
Reset the mikrotik router with default configuration and power it off.

Wait at least 5 minute with the isp modem unplugged. Power on your mikrotik router. The plug back in your isp modem and wait for it to complete it's start up. Wait about 10 minutes and watch the led link status on the ports. Turn on your pc and see it you get an ip address.

If your still don't get an link up on the wan port. Try again. Turn off you pc. Unplug the isp modem and unplug the mikrotik router and wait about 5 minutes. Power up the mikrotik router wait about a minute and plug back in your isp modem. Turn on your pc.

What we are try to do is have the isp modem register the mac address of your mikrotik router.

I believe the default network address on your isp modem it 192.168.100.0/24. I say this because isp modem are mostly setup in router mode.

If you still can't get a link up on the wan port. I would try again but wait about 20 minutes and start over again.

In the past it have ran into this issue. One the isp modem can't negotiate the port speed so I use a crossover cable.
Or issue two would be that the isp modem see the mac address on the mikrotik router as invalid. Then you would have to change the mac address on the wan port to a mac address that the isp modem can register.

You did say that the asus router works. I would use the same mac address and change the last hex value. Your mac address should be on the label on your asus router. Just don't change the prefex mac address. The first 3. You would have to manually enter your ip address on your pc to get access to your mikrotik router which is 192.168.88.1. Go to Quick Set and change the mac address there. For example the mac address on your asus router

Example only! bc:99:11:a2:ba:ea change to bc:99:11:a2:ba:ce

Point being. Use the same mac address on you asus router but change the last two and use the Quick set to change it but first do a reset with defaults on the mikrotik router. You might have to power cycle your isp modem. Not sure about the mikrotik router but I would reboot it too!

Some random ideas:

• cables can not be ruled out with excuse of "they work with other devices" ... physical layout of connectors and sockets has some tolerances and even if they are manufactured within allowed tolerances they still could fail to work correctly (i.e. pins in device socket may not create proper connection with connector pins). It's enough that single wire/pin doesn't make proper connection and the whole ethernet connection fails (or falls back to 100Mbps)
• switch and ISP modem can't negotiate a common speed/duplex configuration ... check switch settings ... on ethernet port, and verify that autonegotiation is enabled and all speeds / duplex variants are enabled

It Is still not at all clear ( to me ) if the issue Is just the DHCP client or something else.

It Is still not at all clear ( to me ) if the issue Is just the DHCP client or something else.

OP stated that port shows "no link". Which means that there's problem on layer1 (physical connectivity) or layer2 (ethernet connectivity). Until this part of problem gets resolved, layer3 (e.g. DHCP) can't start to work.

I'd like to see statuses from CLI. Screenshot from the last post by @Kaszanas shows advertised speeds of ether1 ... many are useless (anything faster than 2.5Gbps ... which is, as per specs, speed of RJ45 ports) and a few are missing (all 100Mbps variants). And I don't know why those are missing (could be that status window has to be scrolled or that those speeds are really not advertised).

So in essence I'd like to see output of /interface/ethernet/monitor [find name=ether1] which should show something. Also log lines with timestamps around the moment when ethernet cable between ISP router and CRS310 is connected on both ends might show something.

It Is still not at all clear ( to me ) if the issue Is just the DHCP client or something else.
I would try the following experiment:
1) connect ISP router->dumb switch->Mikrotik (ether1)
2) the ether1 should get an IP address and connection established
3) check which ip the ether1 got, with /ip address print
4) physically disconnect the Mikrotik from the switch
4) assign manually that same ip to ether1
5) disable the DHCP client
6) connect the Mikrotik directly to the ISP router

What happens?
If the issue Is the DHCP, It should work in the same manner It did when there was the dumb switch between the two routers.

If It doesn't, It Is possible that the issue Is with MDI/MDIX, though It would be a very rare case, It Is a possibility, at least in theory.

@Kaszanas
Did you do this?

Also, go get a crossover cable!

The closest manual I can find

GPON terminal EchoLife HG8010/HG8110/HG8240/HG8245/HG8247/HG8447 (hereafter
referred to as the HG8010/HG8110/HG8240/HG8245/HG8247/HG8447) is an indoor optical
network terminal (ONT) designed for home users and small office and home office (SOHO)
users. This document provides the appearance and specifications of the HG8010/HG8110/
HG8240/HG8245/HG8247/HG8447, and describes its configuration and usage, which helps you
know the HG8010/HG8110/HG8240/HG8245/HG8247/HG8447 quickly.

Description:
Huawei EchoLife HG8347R is an ONU client terminal designed for use with FTTx GEPON networks. This device is equipped with four Ethernet ports. Three of them work in Fast Ethernet 10 / 100Mbps, the last one is Gigabit Ethernet 10/100 / 1000Mbps. With the 802.11b / g / n module supporting MIMO 2×2, there is no need to connect additional access points to provide access to the Internet without the hassle of cables.

Try using port 4..

Like I said. Since you don't have access to the Huawei router your at the mercy of your ISP and how it's configured.

It Is still not at all clear ( to me ) if the issue Is just the DHCP client or something else.

OP stated that port shows "no link". Which means that there's problem on layer1 (physical connectivity) or layer2 (ethernet connectivity). Until this part of problem gets resolved, layer3 (e.g. DHCP) can't start to work.

I'd like to see statuses from CLI. Screenshot from the last post by @Kaszanas shows advertised speeds of ether1 ... many are useless (anything faster than 2.5Gbps ... which is, as per specs, speed of RJ45 ports) and a few are missing (all 100Mbps variants). And I don't know why those are missing (could be that status window has to be scrolled or that those speeds are really not advertised).

So in essence I'd like to see output of /interface/ethernet/monitor [find name=ether1] which should show something. Also log lines with timestamps around the moment when ethernet cable between ISP router and CRS310 is connected on both ends might show something.