@Téo Richoz
Thank you for your post!
Since you have a new phone and no longer have the MS Authenticator App installed, and each time you try to login, you're being told to authenticate via the app. I'd recommend re-installing the authenticator app, adding the account you're trying to log-in with. Once logged in you can disable MFA within that specific account. For example, if you added a Google account, you can enable MFA for that account using this doc - Add your Google account. However, if you want to disable MFA, you'll just follow the same steps, but turn off MFA.
If you have any other questions, please let me know.
Thank you for your time and patience throughout this issue.
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