But you do need to have the correct address of the Modem. It's usually printed on the modem somewhere, with a serial number, mac address, etc.
I have looked all over my router and all it has is the MAC address and the S/N or something.
OMG! I just tried 10.1.1.1 in the browser and it works!!! i still need the username and password though. I tried what Archangel825 said:
"username is Admin, and default password is password" but it didn't work. I will search around for the username and passwords.
I believe you need to find your dsl modem's ip address and change the setting there first, not your router's. It might be easier to call your isp and ask them how to do this (Put your dsl modem into bridge mode)