We are unable to access our old account

We have an account under a new domain, but when we attempt to add our old domain to this account, the system prevents us from doing so, indicating that a Microsoft account already exists under the old domain. Unfortunately, we can’t access the administrator account for the old domain because it requires authentication via the Microsoft Authenticator app, and we never receive the notification or code.

We’ve tried multiple times to add and remove the old domain account, but we still don’t receive any authentication prompts or valid codes. As a result, we’re unable to include our old domain in our new account.

This is critical for us because we collaborate with our clients using the old domain. We don’t want to inconvenience them by changing the domain they already have on file for their Teams environment and other services. We would greatly appreciate any assistance in resolving this matter.