Find the routers ip address. It's usually 192.168.0.1 or 192.168.1.1
Type the address in any browser and if the above ip addresses don't work, open the command line and type: ipconfig /all
The default gateway address will be the local ip address of your router. Type the ip address in any browser and sign in. If you've never accessed the admin panel the default username and password will need to be used (it is usually admin + admin). If that doesn't work you will need to check your router to find the default username and password combination.