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Apologies for the delay in responding here.
I understand you have tried multiple things but the issue still persist, so it can be frustrating.
Typically, the error message indicates either that the credentials configured in Notification Hubs are invalid or that there's an issue with the registrations in the hub.
We only support FCM/GCM through Firebase and iOS/Apple through APNS.
Kindly review the APNS side of things. Kindly review the app id/key and then re-try (if you haven’t done already). Or you could delete this registration and let the client re-create the registration before sending the message.
You must add platform credentials to the Azure portal. If no notifications are reaching the device, the first step is to ensure the correct credentials are configured in Notification Hubs. The credentials must match the application that's created under a platform-specific developer account.