Can't access router with user and pass

Depending on model and is password recovery enabled or disabled - password recovery may or may not be available.

Once you reset the router to factory you can find login information here:  
     http://www.routerpasswords.com/

Well, yes; you could open it up, attach to the internal JTAG header with a USB to RS-232 adapter and log in as root, and fiddle around to change the router's password.  However, while that avoids a full reset, it's definitely doing it the hard way.  Better to go to the manufacturer's web site and download the manual.  Then see if you've got the default administrator username and password still on the system.

If not, and the username and password really are lost, then consult the manufacturer's documentation about how to do a full reset.  Usually there is a hidden button on the back which must be pushed in a certain sequence.

But do be aware that this will erase all the router's current settings, including its WiFi passwords.  So this is a last resort.  Unless you've got the WiFi SSID and passwords written down somewhere, you'll need to set the SSID on the router, load a new WiFi passphrase, and then go reset the passphrase on everything that connects to the router.

Press and hold the reset button for 10 sec,
user login: admin
password: admin
then reconfigure the modem settings.

Please share details on you router

Router name and model

reset to factory.

Is restore the only option?

Yes, Hard Reset is the only option to reset router login Username and password.