Thanks for asking question! Could you please confirm which TLS versions you are using?
As to ensure a higher level of security, Notification Hubs as disabled support for TLS versions 1.0 and 1.1 on December 31, 2020 (extended from April 30, 2020). And its recommended that you review your all of your applications that use Azure Notification Hubs, to ensure that they use the most applicable libraries and TLS stacks that support TLS 1.2.
Reference: https://video2.skills-academy.com/en-us/azure/notification-hubs/notification-hubs-tls12
Please let us know if issue remains.