There’s usually a “proceed” option you can select to bypass this. Usually underneath the warning. Have you tried this? 
Or a different browser?

Since you are getting the error while accessing the GUI web portal, I would suggest following the instructions given below:

http://arris.force.com/consumers/articles/General_FAQs/SBG8300-Web-Manager-First-Time-Login

If you are getting the same error again as invalid credentials after trying the above instructions, I would suggest to factory reset the modem and try logging in again.