Hi Tom,
In this case you will need to contact the Tenant Admin to reset the account MFA settings for your account and this should allow you to re-add MFA settings on the App this is called re-registration process.
if you are the only Tenant admin you will need to contact Microsoft support to reset the account via the proper validation process https://support.microsoft.com/en-us/topic/global-customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2
Hope this helps.
JS
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