Are you using the native iPad Mail App to access your email - or another App that is no longer compatible with your iPad?
The native Mail App should still be able to access your Yahoo/AT&T mail account - as IMAP and POP3 mail protocols have not changed. However, be aware that your email provider is now very likely to require that you create (and use) an App Specific Password to access your email account.
In common with many other Mail providers (e.g., AOL, Google, Microsoft), Yahoo now use App Passwords to secure access to IMAP or POP3 mailboxes.
The Mail App on each of your devices, such as your iPad, requires a unique password to be generated from within the Yahoo account. This App password intentionally differs from your Yahoo account password - and can only be used to access your Yahoo mail.
More information can be found here:
https://help.yahoo.com/kb/SLN27791.html