A friend of a friend went through this a while ago and I was called on to help. Insta was totally taken over and so I made her start going through the linked gmail account because there should be a message in there from insta about the details changes. Nope. You use the same password, don’t you. Yup.

So it’s - access insta, change email, phone and password. Access gmail, delete notifications from Instagram, profit.

In her case she had about 60k followers. They sent random emails the moment she tried to do a password reset, so they obviously back the email onto an automated random service.

The fact the insta even allows changing all of those details in quick succession is one thing. The impossibility of contacting them next is another. Took a few days but they eventually flipped the account back to her with about as little fanfare as taking it away.