It now works for me… but what I see is that ATT changed the URL for their login page to one that begins with https://signin.att.com… Earlier it was https://att.com…
Before I could get it to work I had to clear both ATT history and ATT cookies in Brave, then quit the browser and restart it.
There was an update because before I did the above I did an About Brave (in macOS) and it made me relaunch.
(I wish Brave would put the date of their releases on their update page:
https://brave.com/latest/ )