You might have to figure out how the connections in the MT3000 are made, as I did for the SFT1200 (Internet divided by 3 by GL-INET - #17 by bpwl1 )
Independent from router or AP mode, the ethernet LAN ports there are connected by a switch. In AP mode the LAN and WAN interface are connected by a bridge. The "repeater" is then also setting the WLAN on the bridge when "extender mode", or "AP" mode is used.
Sometimes it is more convenient to use "router mode" to have access to the setup menus, but only use the LAN and WLAN connections, avoiding the router/firewall/NAT function.
VLAN will be passed unaltered if the switch is in dump-switch mode, however GL.inet uses VLANs to separate WAN and LAN, so it is not in dump switch mode. Can be seen (and maybe it can altered to dump mode or other VLANs added) with LUCI and SSH.