It will be interesting to see what AT&T tells you.
You have Fixed Wireless Internet provided by AT&T from the sounds of it.
This thread indicates that AT&T has a router that sends out the wireless connection to your rooftop antenna. The router in your home then sends out the wireless signal that your computer and other stuff connects to.
If I understand what these guys are talking about, the problem is the upstream router. It changes the IP address frequently which apparently certain secure websites don't like. And that is why the one website you are struggling with might be blocking you.
Read here. I'm sure I don't understand everything being said but it may be useful to you.
https://forums.att.com/conversations...d5f2f6066267cb
Also this thread.
https://forums.att.com/conversations...625bd45#M47292