If you don't remember your settings, you will have to reset your router to factory default. To do a Hard Reset do this:
While the modem router is powered on, press and hold the reset button on the rear panel (next to the on/off switch) with a pin for 10s, and then release the reset button.
Wait for 2 minutes, try to login to the device (http://192.168.1.1) again by the default username and password (admin/admin);
If you don't have the user guide, you can get it here
This is also the place where you can find firmware updates and advanced troubleshooting information (it's called "V2 - Installation Guide" ...don't ask me why)
Since this is an ADSL modem, make sure you have the information provided by your ISP: Username, Password, VPI, VCI and Connection Type or you will not be able to connect to the internet.
Let me know if you have any questions and