Details from before it stopped working is also important the power supply might be faulty or some cables vibrate loose during operation.take to a repair centre if under warranty or a 3rd party repairman
When I got my WiFi router they give you login details that takes you to a website that controls the router all settings are on that website including changing the password range and WiFi signal strength. You should log into that website from a browser if you have that information. Some routers or modems are set at a lower signal or on mixed that changes between the two for best network range.