My Xbox One won't connect to the internet. Says "Additional Authentication Needed."

This is a rather Microsoft Xbox One-specific message.  I would ask Microsoft what the meaning is.  If they can explain what the message means, then maybe we could help you.

I did contact Microsoft. Multiple times. They said to contact the ISP, which is you guys. I already did all the hard resetting, and the router troubleshooting, etc. 

Well, you're on the community forum.  I'm just a customer like yourself, not your ISP.  However, I'd bet many of the AT&T support reps will not know what Microsoft means by this message, so if Microsoft would be so kind as to explain what this message is intended to convey, then perhaps either this community or your ISP could help you.

Basically, as far as I'm concerned, this message is in Japanese, and I don't read Japanese.  If we could get Microsoft to explain it in English (standard networking terms), then maybe we'd be getting somewhere.

oh I see. I thought this was an actual forum where the Support Reps could comment. I don't really know what to do then because Microsoft support keeps directing me back to AT&T Support. 

I understand that Microsoft isn't taking ownership of the problem.  However, please ask them, specifically, what should you ask the ISP to do to fix this message?  What kind of Authentication isn't happening?  A protocol name, anything really, for us to go on.  The message is Microsoft's, not AT&Ts', Microsoft should know what it means.  We don't.  If we can just get Microsoft to explain it, maybe we can help.

Enter the MAC address on the back of your router 👍👍😉

Josh165, I have the same problem and this doesn't help.