It depends on the router and it changes with each model and company. Usually, you should be able to login to your router config page by opening your browser and typing in 192.168.1.1 and then pressing Enter. You will be prompted for a username and password. If you haven't changed any setting, you should be able to login with admin as both the username and password. Then, under the Save and Reboot (or any other similar section) you should find a "Factory Setting Reboot" option. Select it, and then save and reboot your router. Once its done, login back to the router config page and under either PPPoE or ATM settings, you should see a username and password field. Enter your MTNL username and password in and save and reboot the router once again, and you should be all set.