Tenancy Admin Access when old admin has left

I have taken over as Head of IT and am trying to migrate from on-prem Exchange to O365.  The previous Head of IT had created a tenancy on his work email address (to which I have access) and linked the email domain to this tenancy.  SSPR wasn't enabled so can't reset the password.  I've had a ticket logged with MS for 22 days with no response or resolution.

Any ideas how I can progress this?

Many thanks in advance