Use GL-XE300C4 as modem only

I have a main router/firewall device that supports 2 WAN ports. I have a fixed cable internet as my main internet connection and I want to use the XE300 as my 2nd failover WAN connection, but without doing double NAT.
This is my use case.

When I go on holiday, I take the XE300C4 with me as a mobile wifi device and buy a local sim car into it. This way I can walk around and not have to buy 5 sim cards for the whole family. When I take it with me, I would put it back into normal router mode.


OK, got it, thank you very much!