A cloud-based identity and access management service for securing user authentication and resource access
Thank you for your post!
I understand that you're trying to register a custom domain within your Azure AD tenant, but the domain is currently registered with a tenant you created years ago, and currently you aren't able to login to your older tenant's Global Admin account.
As mentioned by @JimmySalian-2011 , since you're able to login to your older tenant, prove ownership of that tenant and domain, and the issue is with getting access to your Global Admin account. I'd recommend reaching out to our Global Customer Service team, or contacting the Azure Data Protection team - (866-807-5850), so they work on getting you back into your Global Admin account.
Note: If you have an active Azure subscription and would like to work with our support team on this, please let me know and I can enable your subscription for a free one-time technical support request so you can get this issue resolved.
I hope this helps!
If you have any other questions, please let me know. Thank you for your time and patience throughout this issue.