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Hi Andrey,Since you are the sole Global Administrator and the issue is an MFA lockout, this should be handled as a tenant recovery case by Microsoft’s Data Protection team. Microsoft moderators have confirmed that this team has the required process to verify tenant ownership and restore administrator access.
Since you already have an open case, reply to it and explicitly request “escalation to the Data Protection/Tenant Recovery team and a duty manager due to a prolonged sole Global Administrator lockout.” Include the existing case number so another duplicate case is not created.
If the case continues going in circles, contact Microsoft Business Support by phone, provide the existing case number, and ask the agent to review the case history and escalate it rather than starting the verification process again.
Also, I recommend removing the support case number from this public Q&A post, as case information is better kept between you and Microsoft Support.
Thank you, and feel free to respond back for more assistance.