Very simple. You need to reset the router. There is small button next to power pin that needs to be kept press for few seconds You may need some pointed object like needle to press the reset button.
Once done then you can login with default ip i.e 192.168.1.1 and default username and password is admin/admin. You can also call ip MTNL tech team to know the default factory password for MTNL routers.
Hope this helps in resolving your issue. Please do reply