For all of those with ATT accounts, I had to get a 'secure mail key from ATT and use it during the email account setup process where it asked for password. Overall, it was a very frustrating process and I was shocked when it finally started working. It really should not be this confusing. I did the following.
1. Got the secure mail key by logging into ATT account and going to profile and requesting the key.
2. Deleted existing yahoo/att email account from iPhone
3. Adding the new 'other' email account using the steps is the article, https://www.serversettings.email/att.net-email-server-settings-imap.php#iphone
*** use email key where it asks for password
Thank you to all that posted some information on the topic because there does not appear to be a 100% clear answer for all situations.