I have two of these RUT240s in operation at customers, but I only use the mobile connection. Switching the internet connection between the WAN port and the mobile network should be quite possible. I also assume that the internet and/or network connection can be released on the basis of the MAC and/or IP address. So far, I have only successfully configured these functions with the FritzBox. Finally: I assume that it is possible, but I cannot guarantee it.

I have the device in my camper, and what you want will not work with the device, what you need for this you will be able to implement with a HW firewall with two Internet accesses (so in your case router). One router (modem from Swisscom, for example) and as a second router/modem (fallback) then the GSM router. Then switch off the DHCP on both modems, which would then be taken over by the firewall. And in the firewall you can then set the rules, what if ....?
But that's not easy, but it's a lot of fun when you get it to work.
Have fun with it.