I'm using a rb951 as a travel router. It creates vpn's and makes sure all my devices communicate over those vpn connections.
I use two setups. One config where eth1 is the wan port, and wlan1 is in ap bridge mode. The other config has wlan1 used as wan in station bridge (or psudobridge) mode and my wifi is provided by virtual slaves of wlan1. The problem is with the later setup - if wlan1 configured as station is not connected to an AP (which it never is when I come to a new location), the wlan slave interface is down.
Before now, this has not caused any trouble because I can connect my laptop to the router using a cable, change the settings and get the wlan running. Now, however, my new laptop does not have a wired nic.

If I configure the wan-wifi on a virtual wlan interface, it fails to connect in most hotels. Is there any way I could make the slave wlans accessible even if the master wlan in station mode is not connected?

No, slave wireless interface can not run if master is not running.

I'd suggest you to return the new laptop due to lack of fundamental functionality (wired network connectivity) but you probably don't want to. So you'll have to get a USB (or TB or whatever wired peripherial connectivity available) ethernet NIC for this very purpose.

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I'd imagine the wlan master interface would be running when it looks for AP's to connect to. Loosing wlan connection is not the same as killing the interface. If the mikrotik wlan master is connected to a AP, and the AP is switched off, the slave wlans on the mikrotik keeps running for a few seconds, then they are disabled. I find this a bit strange, as when you have the master wlan configured as AP bridge, the clients connected to the slave interfaces does not get disconnected if a client connected to the master wlan disconnects. I was hoping for similar behavior when the master wlan was configured as station bridge.

Using this activates the master interface, even without an active connection as station-bridge. The slave WLAN (the AP bridge) appears in the wifi scanners and sends beacons. But as the master interface keeps scanning for its AP the frequency changes all the time (I disabled all the entries in the connect list, so WLAN1 did not find any). So I could not connect to the WLAN2 with a windows PC. "cannot connect to this network"

disable-running-check did nothing, looking from the wireless devices. Slave wlan did not show up in list of wifis.
However the master wlan does keep the running flag, even when not connected. (I'm currently not using connect list. Just set one ssid and one security profile)
If the problem is that it changes frequencies all the time, maybe there is a setting that makes it stop scanning for AP's for a few minutes, enough to get in and change config? In winbox, when running a wifi scan, there is a background-checkbox. Would there be any chance to make the AP scan run in background?

Using ROS 6.48

I'd suggest you to return the new laptop due to lack of fundamental functionality (wired network connectivity)

No way! This is the only portable computer that exist! There are no other laptop out there that provides 20+ hours of effective battery time (unless I do graphics work), with the weight of 1.0kg and dual PCIe x4 SSD's.

See my links before. I do it with adding my smartphone hotspot, Woobm and MQS wifi's to the connect list. Any of these 3 will immediately give a stable master+slave , so I can then connect and add the needed entry in the connect list for the public wifi at that spot. (Just power up the Woobm or MQS it does not need to be physically connected to the router.)

And afaik the RB951 has a USB port, so a Woobm would give console access. Anyway a MQS would give wifi access (and PoE power) via the ethernet port for every router.

Now, however, my new laptop does not have a wired nic.

Have you considered using a USB nic?

Using other devices, like a hotspot mobile phone, usb nic, console cable with usb adapter.... I consider those a workaround, not a solution. A better workaround would be to use a dual radio mikrotik.
Shouldn't be that hard to make a wlan available for config.

A better workaround would be to use a dual radio mikrotik.

Not a workaroud, it should be actual solution. By using single radio both for connecting to wireless network and as access point, you're cripling the wireless network by using excessive air-time (more than twice compared to normal user). I think hAP ac lite (TC) is the smallest/cheapest unit with dual radio.

Not really. Cause when you travel, you very often get to somewhere where the distance from your room to the AP you're connecting is rather long. And then you may also want your private wlan to cover the hotels dining area, or outdoors. The 5GHz just doesn't reach that long. Hence the 5GHz end up being for config access only. I can't really accept buying another product as a solution to configure the one currently in use. And the network speed in such locations are never fast anyway, so having 5GHz doesn't add much functionality.

I'm thinking maybe a script would do. Something like this psudo code to run at boot might do the trick:
#make sure net is up before running script
sleep 10 seconds

# let eth1 become wan port if connected
if eth1.connected == true (
disable bridge port where port == wlan1 & bridge = wan # remove wlan1 from bridge wan
enable bridge port where port == ether1 # assign eth1 to bridge wan

# no need to do any more if wlan1 is in AP mode
if wlan1.mode == AP exit script
set wlan1.mode == AP
set wlan1.ssid == rbconfig
set wlan1.security-profile == rbconfig
enable bridge port where port == wlan1 & bridge = config # assign wlan1 to bridge config
exit script
)

# if we make it here, eth1 is not connected
# give wlan1 some time to connect
sleep 1minute
if wlan1.mode == station && wlan1.connected != true (
# wlan1 is in station mode, and has not been able to connect to AP - make wlan1 an AP for config
set wlan1.mode == AP
set wlan1.ssid == rbconfig
set wlan1.security-profile == rbconfig
enable bridge port where port == wlan1 & bridge = config # assign wlan1 to bridge config
exit script
)

# if eth1 is not connected but is configured as wan port...
if eth1.connected != true && bridge port where port = eth1 is enabled (
disable bridge port where port == wlan1 & bridge != wan # remove wlan1 from bridge config or other bridges
enable bridge port where port == wlan1 & bridge = wan # assign wlan1 to bridge wan
disable bridge port where port == ether1 # remove eth1 from bridge wan)
set wlan1 config for using connect list
exit script
)

# if we get here, there should be a wlan available for doing config - need to change anything

I am running exactly the same setup and have run into the same problem. More, I have tried to set up in reverse: physical wlan1 as ap bridge and virtual wlan as station, and for some reason virtual wlan does not pick up the connection at all in station mode. Which is strange, as this option was advertised in several presentations all over the internet.

Station can only pick connection if master interface radio settings (most notably frequency) align with radio settings of AP client is trying to connect to. If settings are left to default, then chances are slim. If, OTOH, both remote AP and master interface have those settings set by hand and intentionalky so that they match, then your setup (slave interface in client mode) is possible.

What usually happens (master interface is client) is that client searches for AP with defined SSID making radio settings changes as needed. Slave interface can not change radio settings of master interface however. For this very reason it's normally not possible to have single radio to act as client to two different APs at the same time, even if radio parameters are left at auto settings.

Thank you mkx, now everything makes sense. So this is a hardware/platform limitation.