Okay. Obtaining an IP address would be one of the last parts of the PPPoE authentication.
I assume, since you specifically stated it, that your username & password is indeed correct... but does your router have any logs that will tell us whether the authentication was successful? Because if this is the case, then it's having an issue after the authentication, but still inside the ppp auth.
If you plug one of your computers directly into the modem, and configure it for PPPoE, can you authenticate authenticate, get an IP and surf the Internet in that configuration?
Also, try updating the firmware of your router and modem to the latest.