There are actually at least two posts about the VLAN problem on this forum (search “b1300”). In short: I need to trunk two VLANs (100 and 110) to a single physical port and bridge the two VLAN interfaces to one WIFI ESSID each, so traffic from each ESSID goes only to the attached VLAN and leaves the device via the trunk port. I cannot use two physical ports (only one cable) but don’t need the other two physical ports.

There is a device from ASUS using the same internal switch setup which seems to have the same problem (search OpenWRT forum) as well as a two years old patch for the driver which however never made it upstream.

UPDATE: After trying again I think I managed to set up working VLANs.