To change the default WIFI name and password you need to follow the steps mentioned on the below article and If your unable to access interface you can reset modem then try the steps to access modem interface to configure the details. You ca also do this on surfboard central app.

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Changing-the-Wi-Fi-Name-and-Password

I am trying to reset default Wi-Fi and password. I am using a Windows 7 version of Firefox. Site says though version is not supported I can continue with possible limited usage of site. However i am not even able to log on. Why can’t I even logon? I can see Software and Wi-Fi Status boxes.

Since you are unable to get into the GUI web portal. Could you try with different device and different browser.

Also make sure that there is no VPN enabled on the devices.