New Arris SURFboard SB8200 modem connects but no internet

Hi, we went out of town for the weekend and turned off our modem & router. Came home yesterday, turned it on. All the light came on as usual, looking good but .... no internet! After a lot of troubleshooting, I decided it must be that our older model modem was no longer supported  by Xfinity.

Went out today and bought the Arris SB8200 which Xfinity website says is supported. Hooked it up, contacted Xfinity to activate. All looking good, all lights on and steady. But ... no internet! I tried both via the wi-fi router (connecting with both a phone and laptop). Also tried plugged directly into the modem via ethernet. Nothing.

The weird thing: when I connect via ethernet cable directly, and go to the modem's browser interface, it all looks good and connected. Signal strength ok. Green lights all around. Connected connected connected. But still .... no internet.

Help!!