Did you receive a message that the registration was successful after you registered? And did you try to log in in a new browser after completing that?
If it's still not working you should test your on-premises firewall settings. Make sure TCP/UDP 443 and TCP/UDP 5671 are open.
Otherwise, if all of this is fine, it is likely that you either need to 1.) update to the latest health agent; or 2.) Ensure that data freshness alerts are active on the service https://learn.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-health-data-freshness
Hope this helps!