In this case we would suggest you to try on different web browser or different device or use Incognito Tab if still you are unable to access interface you can try performing hard reset on the modem and try accessing interface.  To perform please, disconnect the coaxial cable from the back of the device? At the back of the modem, in the middle section you will see a small hole, inside that hole there is a button, and that is the reset button, please insert a paper clip or a tip of a pen and hold it for 15-30 seconds until all your lights go off, afterwards connect the coax cable back and wait until your device fully boots up. But after resetting the modem will go to default setup any custom settings will be removed so use the default network name and password to connect to your network.

http://arris.force.com/consumers/articles/General_FAQs/SBG6700-AC-v8-6-x-Admin-Login-Setup

Thanks. I’ll give that a try.