In your computer, go to options of the Settings application
2. Go to the options of Mail, Contacts, and Calendars.
3. Enter the button of Add Account.
4. Click on the options of Other choice.
5. Choose the options of Add Mail Account with the help of drop-down menu options.
6. Enter your name, email address, password.
7. Choose whether or not you want to sync both Mail and Notes.
8. Enter the button of Save. Your AT&T email is now available on your mobile device.
Follow above given steps to Add ATT Email in iPhone.
See Also this post:- https://www.datarecovo.com/att-net-email-login/