@Shuvagata 

Hi,

If that is the case, then you can try if you can still access the router's admin webpage by connecting a computer via LAN cable to the router and then change the Wi-Fi password and the admin password to something new.

If you can't access the admin webpage like that, then your only option would be to perform a Factory Restore by pressing and holding (> 5 seconds) the physical reset button of the router and then configure the router again from scratch. Again, make sure you choose a new Wi-Fi password and admin password that nobody but yourself knows.