GL-MT300N-V2 - Modem/Passthrough Mode?

Morning all, i’m trying to create a scenario whereby the Mango router acts as a ‘modem’
Currently i’ve tried going into advanced mode, installing luCi and bridging the eth1 to eth.01 (LAN) with no luck, similar to this it seems:

Basically I’m trying to setup a secondary WAN link for my Unifi UDM. The 4G dongle I’ve installed detects as a tethering device. unfortunately it seems to be setting the interface of the LAN port to 192.168.0.*