The most probable answer, is you need to reclaim your account back from Microsoft's data protection team.
The second part of the advice would be the one you need from the post below
https://answers.microsoft.com/en-us/msoffice/forum/all/reset-device-with-authenticator-app/f823c3d6-7e5b-4e96-bba5-f19dfe676118
However, if you sit on the side of the account protection team, they would be caught in a difficult situation. If they were to contact the official administrator, no doubt the person would paint a different story. They would have no way to know who is telling the truth. In such an case, you might be asked to produce a police report / court judgement or similar document to show that you are the rightful party to claim the account. Not an easy journey. First step is to try to contact the account protection team first.