This is a nightmare! I spent hours on the phone with xbox, my phone company, and my router company several months ago getting everything set perfectly so that my NAT settings were finally fixed the proper way. Now all that hard work is ruined!
Here is my error message when I go to the network if it helps.
After you run "Test Xbox LIVE connection" press “Y” for "More Info”. In the pop-up that appears, what are the following values, if shown? (If a value below does not appear for you, just leave it blank.)
D:(my last name here which I am withholding)