Sonic.net

After connecting the the gateway's status/config web page, under Home Network -> Wifi, I can see:

Guest SSID
Guest SSID Enable [Off]
Network Access [Internet Only]
Guest SSID Subnet [192.168.1.0/24]
Guest Network Name [ATTtbvapBP_Guest]
Password [ ]

Is it as simple as toggling Enable, filling in a new network name and password? I want the simplest possible setup; I've done nothing else but change the main network name and password.

That should work. It will create a guest network so devices connected to this guest network are isolated from your network and on a different subnet.
I don't know about the nokia, but some systems require to "apply" the changes so you can do multiple changes, and then apply them all in one operation. Until applied, they are not effective.

Worked fine, thank you.