Your “modem” interface needs to work like what would ordinarily be your “WAN” interface. That probably means creating a firewall zone for it if it doesn’t have one already, and setting up forwarding and masquerade on it similarly to what is on “WAN”. That’s probably easiest in the “Advanced” config (LuCI). The WAN interface can be left there, as “it isn’t doing any harm” if it doesn’t have a DHCP server to give it an address, and doesn’t have link (no cable plugged in).