iMac fails to connect to AT&T U-verse via ethernet

I'm not a noob when it comes to my mac, but I don't know much about networking.  This problem has me really stumped:

For months, my iMac was connected to my U-verse router via ethernet, by way of my time capsule as a router.  Connecting to the internet via time capsule airport works fine.  No problems there.

Now, starting about a month ago, my iMac won't connect to the network.  When I run network diagnostics, "Ethernet" is green "Network Settings" is Yellow "ISP" is green  everything else is red.

In my Network control panel, under ethernet, my status is connected.  It says: 

"Ethernet has a self-assigned IP address and will not be able to connect to the Internet."

Configure IPv4 is set to "Using DHCP."

Things I've tried are:

-Restarting my computer and both routers.

-Creating a new network location under prefs and starting from scratch.

-Setting my time capsule to "bridge mode."

My setup again is:

2011 iMac

Mac OSX 10.6.8

ethernet to a time capsule

timecapsule ethernet to U-verse router

ethernet connection is down

airport works fine

Thanks for any help you can provide!