How can I get my airport extreme to communicate with our new Arris Modem/Router from Comcast - we already have the Arris on Bridge Mode

We were in the local apple store replacing the hard drive on our old IMac....when I explained that we were having trouble with our Airport Extreme, the gentleman at the genius bar asked if we were running two wifi hubs ( the apple and the comcast )...we were, he suggested we contact comcast and have them turn our wifi hub off, so we could use the apple extreme airport base....I did that tonight - they put the Arris cable modem/router on "bridge mode", but now I can't get on the internet...