IP passthrough in OpenWrt

Maybe using trelay? It is basically a brainless packet mirror.

trelay relays ethernet packets between two devices (similar to a bridge), but
without any MAC address checks. This makes it possible to bridge client mode
or ad-hoc mode wifi devices to ethernet VLANs, assuming the remote end uses
the same source MAC address as the device that packets are supposed to exit

you bridge the two LAN and WAN interfaces of the modem device with trelay

And then you spoof the mac with macchanger or whatever other tool (or UCI as said above), on either the modem wan or the router wan so that both ports have the same mac address.