since you tell us that the modem works fine when connected directly..lets try this approach
make sure the physical connection is correct thats the first thing (assuming its all pluggin into the right ports) then try to access the firmware, since you try to set the connection on bridge mode did you put the router on PPPoE mode settings, bec if you put the modem in bridge mode all authentication will be coming from router then you type the right credentials in order to make it work like username and password depending on the ISP..let me know if this approach worked for you.