@NetGobblerCan you post a screenshot of Interfaces -> Assignments ?
Not sure if this helps, but it took me while to find a config my modem and OPNSense were both happy with. My ISP uses PPPoE and bridge mode is acheived simply by setting the OPNSense WAN interface as PPPoE and entering the credentials, the modem then passes this straight through.To get access to the modem's web GUI a second physical cat6 jumper was connected between OPNSense and the modem (which has 4 LAN ports). A MODEM interface was then created and the second physical connection assigned to it. A firewall outbound NAT rule of translation type "interface address" was added to provide access between MODEM net and LAN net.This seems to work really well, despite being a little clunky with the two physical connections.