I suspect that AT&T made a change, because the wife and I both recently had the same issue using Yahoo Mail in Safari. She is running Mojave and I'm running Catalina and Big Sur on different Mac's. Needless to say the wife, never having used the Mail app wasn't to happy at first. Although after a couple of days, she did admit that she wasn't missing all of the AT&T ads.
If you open the Mail App, you should be able to select Yahoo Mail and then log into to your AT&T account.
https://support.apple.com/guide/mail/add-or-remove-email-accounts-mail35803/mac