Are you using the new network's broadcast name (SSID) ? I understand that the Netgear extenders add some letters / characters ?
You don't want to "add it" to your Comcast gateway SSID. You want the TV's to now associate with the Netgear extender. Check the manual(s).
so I need to go through the tv and not the xfinity box to select the network?
Yes. Check the manuals for both the TV's and the extender for instructions.