synced iphone with dead account, now I can't sign in

Picked up an iPhone 8 from a friend and accidentally synced it with an account that no longer exists - except on my computer. It wouldn't sync with my new iPad so I went with my old iPhone account instead. Now when I try to sign in it tells me that the account doesn't exist. I used the correct old password but it eventually just locked me out of the phone. Any suggestions?