I had a SB8200 and the default login was admin and the last 8 digits of the serial number.  Try the following steps when you can bring down the Internet. Good luck.

1. Disconnect the coaxial cable.  
2. Factory reset the SB8200 by holding down the reset button for 20 seconds.
3. Log in with the default password.  It’s either password or (last 8 digits of serial number).  Username should be admin (all lower case).
4. Create a new password.
5. Log out and log in with the new password.
6. Connect the coaxial cable for Internet connection.
7. After Internet is up, log out and log in with the new password.