Using Classic Outlook on Windows for personal email, calendar, and contact management
Since the issue is occurring in both Outlook desktop and Outlook mobile, the first thing to verify is whether you can still sign in directly to your AT&T webmail account. If webmail works, try removing and re-adding the account in Outlook using AT&T's current IMAP settings and a Secure Mail Key instead of your regular password, as AT&T requires this for many third-party email apps.
If authentication still fails after re-adding the account, the issue is likely on the AT&T account or mail-service side, and you would need to contact AT&T support for account-specific assistance.