Earlier AT&T and Yahoo Mail accounts were merged but later in November 2017 they were unmerged and due to this user is not able to access their AT&T or at.net with their Yahoo ID. So the main question is how to unmerge your Yahoo and AT&T account.
Unmerging Yahoo email from AT&T requires some steps which are as follows:
Open a web browser and sign in to your AT&T account with correct login credentials.
After logging in, select “Profile” under “MY ATT” from the top navigation bar.
Under the “AT&T Email Accounts” option, click on “Account Profile”.
Click on “Delete Account” located at the right hand side of “Member ID” by navigating to the “User Information”.
Click on “OK” to confirm the deletion of the confirmation message on the screen.
Now visit Yahoo Mail and on the unmerge option to use Yahoo Mail separately.
For more info, you can either visit the Emailsdesk website or you can visit the official help center of Yahoo.