How to restore the router settings on linux?

In order to get help, you need to describe:

1. what is your router (a separate device or another computer that performs functions of the router);
   
2. What is the OS on the router;
   
3. whether you have access to it (whether you know the administrator password, access via the web interface, ssh, telnet, or physically);
   
4. What do you want to configure / restore ...

Otherwise, you really need a clairvoyant, because you have not told us anything.

The easiest way is to reboot! Well, then you have to re-configure everything. If you do not fall off (it happens), by calling the provider for the 1st time, and talking about your problem, it’s not too bad to know your ip, mask, and dns — that could be prescribed them in manual, if necessary! If asked, there is no way back.