Sole Microsoft 365 tenant owner has lost Global Admin access and cannot open a support ticket

I am the sole owner of a Microsoft 365 business tenant and the associated custom domain. There are no other users or administrators who can restore my administrator permissions.

I can sign in successfully with my existing account, but Microsoft 365 no longer recognises the account as an administrator. When I try to access administrative functions, I am told that the account does not have administrator privileges.

I have tried the internal admin takeover process through Power BI, but the takeover wizard does not complete/offer the takeover.

I have also tried to open a Microsoft support case. The support request fails with:

“httpsupportworker.request internal error create ticket 400”

If I use the Microsoft online support route, it tells me to contact my administrator so that the administrator can contact Microsoft. This is impossible because I am the sole owner and there is no other administrator.

I have also tried both published UK Microsoft support telephone numbers, but the automated system will not transfer me to a support representative.

The Microsoft 365 subscription may have lapsed, which might be relevant, but I can still authenticate to the account.

I need Microsoft to help recover Global Administrator access to the existing tenant. I can prove ownership/control of the custom domain, including through DNS verification if required.

Does anyone know the correct Microsoft escalation route for a tenant with no accessible Global Administrator, where the normal support-ticket system itself requires administrator privileges?