If this is an Xfinity Wireless Gateway and the default userid/password don't work, call the gateway support folks at 800-363-2416 and have them change it so you can log in, then change the login password and wireless setup as you like.
The "Factory" default is definitely admin and password. I just set up my new equipment and had to call Comcast to get help as their documentation said to use 192.168.0.1 (the one to do the initial setup).
Give that number a call from the previous response. The first guy I talked with was very helpful in getting me up and working. Wish I'd written his name down for further help.
If you are not sure of the ip address of your router then ...
On windows, open a command prompt(start dialog -> cmd) run the command ipconfig /all - Look for the default gateway address .. 192.168.x,x, 10.x.x.x whatever and then open up a web browser to that address.
you may have an ipv6 and an ipv4 address for the gateway device - use the ipv4 one.