Switching over to Comcast from U-verse

I thought this was going to be an easy switchover ... sort of unplug the U-verse modem and plug in the Comcast modem. But that hasn't worked out.

I have an Airport Extreme base station 802.11n connected by ethernet cable to the U-verse modem. Things worked OK with this setup. My MacBook Air, 3rd gen iPad, and iPhone all connected well with wifi on this setup.

And I've got a Motorola SB6141 eXtreme Cable Modem connected to the Comcast cable that was installed yesterday. There is a good TV signal from the Comcast cable, so I presume there might be a good internet connection on that same cable. I ordered both TV and internet.

But when I connect the ethernet cable from the Comcast modem to my Airport Extreme, I'm not getting a wifi signal anymore. Airport Utility shows an orange light and that the internet connection is "Disconnected", and also shows the Airport Extreme with a green light and some IP addresses, serial number and version info, along with a few wireless clients listed.

The Comcast cable technician was in a rush to get out of here. He connected the cable to the Motorola modem, checked that the TV did have a picture, tried and failed to get the Comcast remote control to work, complained that the folks back at the office weren't helping him, and got out of the house as soon as he could. The Comcast guy who called at 9:00 PM said they don't know anything about Apple equipment and that I should contact Apple for instructions.

I did find some advice to perform a factory reset on the Airport Extreme. I did that. Plugged things back together, powered up and connected to the Comcast modem. Same problem.

Obviously, I don't know much about setting up a wifi network or getting this mess straightened out. Some help for "Dummies" would be terrific! Thanks very much.