If you are using Yahoo and AT&T merge accounts and your Yahoo email got hacked so you need to change your Yahoo password,  and you start facing issues because your Yahoo account is still linked to AT&T email. there is no need to worry to solve your problem: you need to unmerge your Yahoo account from AT&T email account, after you unmerge Yahoo and AT&T email accounts, you can use both email accounts.

See also - https://www.geeksscan.com/what-is-the-procedure-to-unmerge-att-and-yahoo-mail-account/