Every router has its own interface, which allows you to set a password, control who can access your network and how much can stay on it, etc.
The first thing you need to do in order to access the router's user interface is to connect to the router (via Wi-Fi or Ethernet cable) open a web browser and type 192.168.01.1 or 192.168.1.1 in the browser's search box and then type the router's username and password. However, not all routers have the same usernames and passwords.
Here you may find the most frequent default usernames and passwords for the most common router manufacturers. If they didn't work for you, then they have been changed by someone else, and you need to reset the router and restore the default settings (the username and password you received by your ISP to connect to the Internet, will also be deleted from the router).
Note: If 192.168.0.1 didn't work for you, try 192.168.1.1 and vice versa.