I have been using ATT Uverse and Xbox Live for a long time now. Always had an open NAT using UPnP through the Uverse modem. I recently upgraded my speed through Uverse and they gave me a new modem. It is an ATT NVG589. The NVG 589 does NOT support UPnP. With that being said when I connect to Xbox Live, I can connect however I always get a moderate NAT. This is with the Xbox set to Automatic in the network settings.
I then proceeded to create a static IP for the Xbox 360 and forwarded the Correct Ports. I can still connect to Xbox Live but NAT is still moderate at times. Sometimes it is open.
Furthermore. When I boot up my Xbox it does not automatically connect to Live now. If I try to sign into Live manually it gives me an error about the internet connection. I then go into my network settings. Test the connection. Everything tests good. Then I can sign into Live. But I cannot sign in as soon as the machine turns on without doing a "Test Connection"