Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Ok. my understanding is this - we use contoso as example.
Contoso.onmicrosoft.com - the admin tenant is properly registered. Correct?
You have purchased contoso.com domain and have access to the contoso.com DNS. Correct?
However, contoso.com is registered to another tenant and you are not able to associate it with your tenant, Correct?
In such a case, have you raised a support ticket inside your Microsoft365 admin portal about this?
I believe they are able to advise.