Excuse me, but I want to understand better. Do you want to have the connection to the router as a backup? I see the interface Mobil MOB1S1A1 active and working.
Regarding your WAN configuration. You need to know the LAN IP of the router, I see you did not set it in the diagram. Then the WAN IP of your RUT240 must be on that same network to be able to communicate, and it must be different from the LAN IP of your RUT240 so there is no conflict. Can you check what is the LAN IP of your router to better orient you?