I'm using some RB751G as accesspoints using the default CAP config in CAPsMAN Forwarding Mode (datapath.local-forwarding=no).
I have set two SSIDs (main and guest) and assigned two different VLANs to them.

Now comes the problem: In this device default-config all ethernet ports are bridged, but I want the devices connected to the ethernet ports to be in the same VLAN as the main SSID.

There is no hint in the wiki page how to get this working:
https://wiki.mikrotik.com/wiki/Manual:C ... with_VLANs

In the wiki example above, no pvid is set for the ethernet interfaces (to which the caps are connected) on the caps-manager!
Would it be sufficient to set the bridge port interface to the same VLAN as the caps interface (see below)?

If i understood right, you just have to set your eth1 as tagged and then at your AP side you must create an interface VLAN for that VID...
But why would you need to do that ?

If i understood right, you just have to set your eth1 as tagged and then at your AP side you must create an interface VLAN for that VID...
But why would you need to do that ?

I cannot send tagged VLAN to the AP as I have some dumb switches between the CAPS manager and the AP.

If they are indeed unmanaged switches i am pretty sure they will just forward everything and they will not drop any tagged traffic...
But as i asked in my previous porst, why you want to do that ? You can just configure your VLAN on capsman and thats it..

If they are indeed unmanaged switches i am pretty sure they will just forward everything and they will not drop any tagged traffic...

If they're brain-damaged, they might drop some VLAN-tagged frames ... those exceeding size of 1500 bytes. Most modern dumb switches support MTU of 1504 bytes (4 extra bytes needed for 802.1Q tags) though.

If they're brain-damaged, they might drop some VLAN-tagged frames

Nice to know that

But as i asked in my previous porst, why you want to do that ? You can just configure your VLAN on capsman and thats it..

I really want to avoid any additional configuration on AP side, so that even when it gets unplugged and a normal PC is plugged into the socket it would be inside the main vlan.

even when it gets unplugged and a normal PC is plugged into the socket it would be inside the main vlan.

I would just advise you to make a better network design...

Overall I concur and putting a managed switch vice unmanaged switch in between is an easy change other than $$cost.
However, looking at the brief snipped of code provided that should work, but since I cannot see the whole code and no mention of the other vlan, its too difficult to assess............
My advice would be to try and remove capsman and get the vlans working first and then introduce the added complexity of capsman for what I think is little gain, unless you have 3 or more APs.......