Just a suggestion, I think you should try Admin for the username and the password on the bottom of the modem.  If you’re having problems, you can always do a reset using a paperclip to press in the button on the back of the modem..  I will paste a reply in this community by TechRep that pertains to resetting.    Hope this helps

First, my best recommendation is to disconnect the coax cable from the back of your SBG8300. Then, make sure you are using a suitable item to do the reset (paper clip, toothpick, tip of a pen). Make sure you feel the reset button being pressed (reset button located above the 4 ethernet ports) hold down this button for at least 10 seconds, SBG8300 will reboot if after those 10 secs you do not see any changes on the lights maybe you can try to press it for a little longer. Wait until the first light starts to blink then reconnect the coax cable, after this you should see the default wifi names (the ones printed on the label) showing up on your devices.

If you want to customize your wifi credentials after the reset you can follow the steps in the link below:

https://arris.my.salesforce-sites.com/consumers/articles/General_FAQs/SBG8300-Web-Manager-First-Time-Login

If the GUI  page asking for Email address and password, you can locate the credentials in the Surfboard central application.

1.Open the SBC application

2.Select menu bar.

3.Select access web interface option.

4.Scroll down the page, you can locate email address and password. 

5.Use the same to login the router 192.168.0.1.