Office 365 tenant Admin account access lost

Hi Tech Friends,

This is for one of our customer/client.

Our customer is having on-premises exchange 2016 and they have/had one office365 tenant created by one CSP service provider/reseller for Office 365 for 2 licenses.

The reseller who created Office365 tenant (@dxxxxxb.com) had kept all the access to themselves and never gave any username password access to the client as they said it's a managed services. Later on some disputes happened between client and reseller and reseller deactivated their licenses and also stopped any communication with client.

Now the client who is rightful owner of the domain and thankfully having access to Domain DNS would like to purchase new Microsoft 365 Business Premium licenses while migrating exchange users to Office365 and also reactivate old licenses but unfortunately they don't have access of tenant Admin account and even don't have any idea which email ID reseller had used while creating original tenant.

As per my knowledge for the same domain (@dxxxxxb.com) it's not possible to have multiple tenant ID or administrators, so is there any way where Microsoft can remove previous Admin account and help to activate tenant for the same domain with new admin account?

Or is it possible to have multiple tenants admin account for the same domain and then remove old admin from Admin panel of Office365?

I need help very urgently as I need to setup this weekend for migration.

Thanks in advance.