I don't have those specific router and modem, so I can't comment on the specific setting on them, but your modem should have a setting to either:
- Attempt to login by itself (modem is doing the connecting, and then sharing internet access)
- Allow Pass-through (allow Router / Computer that does the login, and the modem only simply passing through information) - Which I used for my own setting here at my house.
You can try to find some info on the modem setting / manual to see if you can set the modem NOT to login and get connection, but simply allow pass through, and then you can set your Router to establish (usually) PPPOE connection.
For the specifics of the setting, you might want to consider the manual of both router and modem, and also the HelpDesk for your ISP (or your ISP help page if any).