Do you have access to another stand alone router? If so, you can call AT&T and have them disable their router so you can use the external one.
No, I cannibalized my old router for its antenna, installing that in my MK808 mini-PC.
I checked my hosts file, and it's a normal Win 7 hosts file, with all lines rem'd out, and no suspicious entries. I opened it in Notepad, and it doesn't appear corrupted.
What can my ISP or a different router do to fix this issue? It appears to be a simple case of the hosting or target site blocking my IP address.