do you have other devices connected to the router? do they work?
if so, you may want to give panasonic a call. You may need to go to the ethernet settings of the TV and give it static IP along with gateway.
Example, if your router is using the usual 192.168.1.* scheme, give the TV say 192.168.1.61
subnet of 255.255.255.0
gateway of 192.168.1.1
Also use this for your DNS 192.168.1.1
this is provided the router is setup to connect thru dsl ok. if other devices can't connect, then give AT&T a call for setup help with router. You just probably have to put your modem in bridge mode depending on the modem you have and then put your login info into your router. If you have one of those modems with a built in router then maybe there are some ports being blocked in the firewall of the modem which is not allowing the viera-cast to work properly.
Good Luck with this!