Username/password invalid surfboard app- SBG8300

In this case for one last time you can perform hard reset on the modem by following the steps mentioned on the below article and uninstall the app then wait for the modem reboot completely and then connect device to your modem WIFI network then install the surfboard central app and setup. 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. If still same issue persist please reach out arris.com/consumersupport for further assistance if the modem is under warranty.

http://arris.force.com/consumers/articles/General_FAQs/SBG8300-Factory-Reset

http://arris.force.com/consumers/articles/General_FAQs/SURFboard-Central-Wi-Fi-Cable-Modem-Setup