For yourself and all the other lost souls making their way through the Internet trying to set this up: made it work thanks to this topic Trouble configuring VLAN on Unifi 6 Lite WLAN AP.
Turns out all I had to do:
- Install ip-all
- Clean up the config (only leave br-lan definitions, loopback etc)
- Add the following to /etc/config/network, reboot, let LuCI do its thing, assign SSID
config interface 'guest'
option type 'bridge'
option proto 'static'
option ipaddr 'xxx.xxx.xxx.2'
option netmask '255.255.255.0'
option gateway 'xxx.xxx.xxx.1'
option ifname 'lan.2'
Close to 3 full nights for a 20 second fix. Sounds about right