Thank you for reaching out to us. As i understand you are facing this issue "Unable to Sign Into Office365 (Azure) Using UPN of Verified Domain", from the above issue description, wanted to check "Whether you have verified custom domain in Azure AD "
Reference article on how to verify the custom domain - https://learn.microsoft.com/en-us/azure/active-directory/fundamentals/add-custom-domain#:~:text=can%20try%20again.-,Verify%20your%20custom%20domain%20name,-After%20you%20register
If the above step is performed, what is the error you are getting while logging to Office 365 portal ?
Let me know if these steps doesnt help, we can connect offline to discuss the issue.