Hi @Gabor Varga ,
I'm sorry to hear that you are locked out! You said that there are no other admins on the tenant and if that's the case you can reach out to Azure Data Protection, who can help you with this. Their number is (866-807-5850)
If there are additional global admins on the tenant they will ask you to reach out to the other admins for help.
Hope this helps! Feel free to email me at AzCommunity@microsoft.com if you have any issues getting this resolved with them.