@Geethu Jose , Apologies for the delay!
Do you receive any specific error message? Could you please share more details on the setup and your requirement?
This article provides an overview of all the operational and diagnostics logs that are available- Enable diagnostics logs for Notification Hubs -Enable the logs to review and fetch more details.
Just to highlight, Registrations from the app backend are useful when you have to authenticate clients before creating the registration. They're also useful when you have tags that must be created or modified by the app backend based on app logic. Go to the Backend Registration guidance and Backend Registration guidance 2 pages.