how setup airport extreme as a local LAN only to share apple tv

I now have an iphone 5 and would like to use the 4G speed and airplay mirroring with my apple tv. But my other equipment will also share the tv occasionally.

Before the iphone 5 came, I just had the cable modem wired into the airport extreme and connected the iPad, iMac, and iPhone 4 via wifi to the apple tv and everything got internet by wifi from the airport express.

The airport extreme and apple tv are the latest versions. The iMac is running Mountain Lion, the iPad is the 2nd version.

I thought it might be easy, so I just went into the Airport Utility > Network > Router Mode: and selected "Off (Bridge Mode)", then unplugged the cable modem from the airport extreme and plugged it into the iMac, then rebooted everything. Upon restart, the iphones and ipad won't connect, but the iMac does have internet and will mirror on the apple tv. Also, the apple tv complains it doesn't know the time.

The airport extreme has a flashing amber light and the airport utility shows error messages regarding no ip name.

How do I set up the airport extreme to do this?

TIA