Purchased a time Capsule for backup drive use only , connected by US b cable. Now the Imac turns off every 15 minutes or so and won't turn back on unless the iMac power is pulled manually

Read the instruction manual, it won't work at all connected in the way you have invented

Didn't know how the Time Cap would work if connected to AT&T system via ethernet cable.

This is the correct way to set up the Time Capsule with an ATT modem/router.......or any other modem/router.......just as the Time Capsule Setup Guide illustrates.

By default, the Time Capsule will provide a wireless network signal when it is setup. If you don't want or need that feature, (the Time Capsule wireless will run circles around most ISP provided modem/routers), it is simple matter to turn off the wireless function on the Time Capsule so that it can be used as a wired hard drive on the network.

The USB port on the Time Capsule is normally used to connect a USB printer....again, just as the Setup Guide illustrates. It also possible to connect a USB hard drive to the port on the Time Capsule. If more than one USB device will be connected, a powered USB hub is needed.