21.02 3700v2 routes broken?

Dear openwrt community,

I have a very weird problem with openwrt 21.02 and ONLY the Netgear 3700v2. With all other devices everything works flawlessly, also with DSA enabled devices (eg youhua 1200) and others (c2600, 1043v5, etc).

  1. I set it up as a dumb AP with LAN + corresponding wifi.
    -> Everything works.
  2. I add a VLAN, define the corresponding network and add a corresponding wifi SSID. It gets weird:
    a) LAN clients, wired and wireless work flawlessly, internet etc.
    b) VLAN clients, wired work flawlessly, internet etc.
    c) VLAN clients, wireless get no IP, i.e. no internet...
    d) Router itself cannot ping anything (neither IP eg 1.1.1.1 nor url eg google.com)...

I tried various things incl changing the MAC address, no success, c) and d) remain. The only thing to remedy d) is to add a static route to lan (0.0.0.0/0 and so on) AFTER the router has booted up (static route in /etc/config/network won't work, as it gets overwritten by the default route coming from the VLAN network, which usually (on all other AP's) has no impact on the connectivity).

I'm out of ideas, can anybody help me ?
I can post the configs, please tell me what to post (network, dhcp and wireless? in other files I didn't adapt anything)

Thanks in advance ! Blinton.

PS: The setup described is just an intermediate step to get to batman-adv (which I know how to do), but if I get problems already now (only with 3700v2, weird device), it gets unpredictable...