So I see the tagged frame from the dhcp request on eth0 but I can't see any tagged frames outside the AP.
Also I do not see any tagged frames from outside on the AP's tcpdump.
The original dlink firmware supports 8021.q vlans.
Any idea?
I have exactly the same problem and see the same behavior on an Aruba AP-303 (similar platform). Seems like VLAN support is currently broken on snapshot releases. Not sure, but I believe this thread is somewhat related.
Your configuration looks fine and would work well on a device running OpenWrt 19.07.
I can check a snapshot image against vlans with my raspberry when I'm back from work.
I checked the sources from the D-Link original firmware and many vlan related things are precompiled (without source).
Also, the D-Link DAP-2600 uses the ess_edma driver - May be the problems are related to this driver?
Good point, I've just installed the latest snapshot release on a Raspberry Pi 2 and Software VLANs work as expected here. So this is not a general problem with current snapshot releases.
Ok, so it's device / driver related, I think. In fact, with tcpdump I receive never any tagged frames on eth0 so this seems to be blocked by the chipset I think.
Software VLANs are working now on my device with current snapshot builds!
But unfortunately there seems to be another weird bug. When the software VLAN interface (e.g. eth0.10 is bridged to a wifi interface, then the wifi clients can connect and obtain a DHCP lease, but no other traffic passes through the interface - not even IPv6 auto configuration). I will need to debug this a little more.
Edit: It's completely broken, when you set eth0.10 to a static IP, then no traffic passes through eth0 or eth0.10 anymore. Only DHCP is working initially.