My take is you only connect to the TO via WLAN and there is no connectivity once the eth0 is removed fron the LAN iface?
Are you running the LAN solely on IPv6?
Are the clients still able to connect to the internet and only the TO is not accessible or there is no connectivity at all?
Did you reboot the TO after having removed eth0 from the LAN iface?
It seems a bit inexplicable but then the network management package
netifd is outdated and buggy in TOS3.x and this could be just another peculiar consequence.
You could try via ssh edit etc/config/network, there is no option for it in LuCI/Foris:
option type 'bridge'
option bridge_empty '1'
If that does not help this additional step might do the trick:
kmod-dummy and add