the router password to login into the router itself or the wifi password?
To log into the router, if you don't know the password and didn't setup password recovery, you'll need to factory reset. Push and hold reset while the C3700 is powered on. its usually 7-15 seconds or until the lights change. That should reset it back to defaults
default login: admin
default password: password
Keep in mind they are case sensitive.
If you don't remember the wifi password, you can just hardwire into the C3700, log into the router, and get the wifi and password in the wireless settings section.
