Choose the options of Mail application from the home screen.
Choose the options of your preferred email service provider.
Enter the button of Next after entering your email address.
Enter the button of Next after entering your password.
Choose the options of Save after setting the desired account sync options.
The Mail app now has your email set up.
Follow above given steps to Add ATT Email in iPhone 7.
See Also this post:- https://www.datarecovo.com/att-net-email-login/
https://www.datarecovo.com/fix-att-email-not-working-problem/