I don't see why the two interfaces would be on the same subnet. The netmask from both (ISP modem and 4G router) dhcp servers is 255.255.255.0. And where would be an IP conflict if the two interfaces are assigned different addresses and different gateways to the Internet at large?
Really, this use case seems to be the closest to a by the book use case for the multiwan failover mode and still the manual is finally not so clear on how to make it work.