I am trying to restore access to admin account for my customer. Lost MFA

For some time already I am trying to restore access to lost admin center, I have login and password, contact number and contact e-mail but I lost authenticator device.

I am trying to work with MS support but I feel that the subject is stuck.

I need a real steps of what I can do to restore this.

This is my customers Tenant, we can provide all the information that is necessary to restore the access.
Owner of that tenant and Me as support company are trying to reach out for quite a time already.

Please any instructions on how to get support? I feel like phone calls are just dead end. I need clean instruciton on what to do and what to provide.