why wont my iPhone take an att.net email account anymore

I've had an att.net email for years. Way too many people and companies to count have it associated with my accounts etc.

Suddenly, recently the phone would not get mail from my account after years of doing so.

years ago att.net suddenly had me redirected to Yahoo mail. Same exact email. still no problems.

Now after days of frustration following numerous recommendations, I deleted the att account from my phone. Did an update (14. something or other). now I try to add the email account in and Att doesn't come up at all. I add it as "other" and nothing works. I tried adding Yahoo. same. no such email or account.

What the heck is going on? I can access my email at ATT (YAHOO) on my PC by typing ATT.net into the browser. I am redirected to Yahoo and I get my email.... (my Windows outlook account works on PC but suddenly not my ATT account. I must access through the internet page.

again I ask. What the heck is going on?