Recently I bought MikroTik | SXTR&FG621-EA LTE6 (new).
I have hybrid internet from operator Telia and Sagemcom f@st 5360t router (VDSL and 4G LTE). So I removed sim card from Sagem router and put it on MikroTik | SXTR&FG621-EA LTE6. I could see operator, band, signal, but no internet (no ping, no firmware check).
I bought prepaid sim card (the same operator - Telia), put it on MikroTik | SXTR&FG621-EA LTE6 and all working with no problem. Updated router OS and firmware successfuly.
Put back sim card on Sagem router - internet is working. Looked at APN - it was different than default on MikroTik | SXTR&FG621-EA LTE6. Ok... need change APN: removed sim card from Sagem router, put it on MikroTik | SXTR&FG621-EA LTE6 and changed APN. Result the same - internet not working.
And the second step was.... I put that sim card from hybrid internet to mobile phone (Samsung) and after APN change - internet start working. So in result: internet with changed APN is working on hybrid router Sagemcom f@st 5360t and on Samsung mobile phone, but not working on MikroTik | SXTR&FG621-EA LTE6. Any other suggestion are welcome..?

What FW version is modem and which ROS version on SXT ?
Does LTE interface remain visible or does it disappear ?

Could it be related to this problem (same modem) ?
viewtopic.php?t=204276

Long story short:
There is a problem with FW version 05 for that modem when APN is used with "Use Network APN". It results in a FW crash (hence no LTE interface and no internet).
Can you create a new APN using the same settings but deselect "Use Network APN" ?
Does it work then ?

LTE interface is always visible and always show RSSI, RSRP and other two. Prepaid sim card worked all the time (about 2-3 hours).
I have tested default settings (router and firewall, with NAT), only router and firewall, passthrough to eth1 or eth2, used and not used APN, changed APN settings (e. g. auth). Nothing helped - internet not working.
At this time I can not check firmware version, I will do it later. But i have done all possible updates with prepaid sim card.

Okay, you have at least a registered LTE connection which is a good start. Run these commands so we might see where it cracks. Btw, feel free to mask out any public ip address if not NATed.

status: connected
model: FG621-EA
revision: 16121.1034.00.01.01.04
current-operator: Telia
current-cellid: 46601495
enb-id: 182037
sector-id: 23
phy-cellid: 217
data-class: LTE
session-uptime: 3m28s
imei: xxx
imsi: xxx
uicc: xxx
primary-band: B1@20Mhz earfcn: 300 phy-cellid: 217
ca-band: B28@10Mhz earfcn: 9360 phy-cellid: 268
rssi: -47dBm
rsrp: -73dBm
rsrq: -14dB
sinr: 0dB


Flags: D - DYNAMIC
Columns: ADDRESS, NETWORK, INTERFACE
# ADDRESS NETWORK INTERFACE
;;; defconf
0 192.168.188.1/24 192.168.188.0 bridge
1 D 100.75.30.120/32 100.75.30.120 lte1


Flags: D - DYNAMIC; A - ACTIVE; c - CONNECT, m - MODEM
Columns: DST-ADDRESS, GATEWAY, IMMEDIATE-GW, DISTANCE
DST-ADDRESS GATEWAY IMMEDIATE-GW DISTANCE
DAm 0.0.0.0/0 lte1 lte1 2
DAc 100.75.30.120/32 lte1 lte1 0
DAc 192.168.188.0/24 bridge bridge 0

# 1970-01-02 00:08:52 by RouterOS 7.13.5
# software id = NU11-ZF6U
#
# model = SXTR
# serial number = HF8094XASX8
/interface lte
set [ find default-name=lte1 ] allow-roaming=no band="" sms-read=no
/interface lte apn
set [ find default=yes ] apn=xxx

It works! I don't know how, bet manual set date ant time and uncheck "Use Network APN" again. And internet start working.... Nice...
My operator doesn't give me external IP. So i think it's ok.

Great, well done! Regarding the lte1 ipv4 address you are correct as the 100.75.30.120 ip is a CGNAT address. Enable IPv6 to obtain a public ip address (IPv6 GUA).

Have also seen some ISP, don't want there customer use a router just use a modem.
They check that with the ttl, so a workaround to that will be to mangle the ttl.

Now you can press this bar to mark your thread as solved.

They check that with the ttl, so a workaround to that will be to mangle the ttl.

Yep, but you do realize how this sounds as Vogons poetry to many, don't you?

Just in case:
viewtopic.php?t=171544
viewtopic.php?t=171544#p839136