502 - error code in azure when I am trying to send a push notification to iOS device

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C#

.NET

WebForms

webservice

I have uploaded a web page to azure server.
It has been created in .net framework.

This page sends a push notifications (APNS) to perticular mobile device with reference to REGS ID of a ios device.

but whenever I am submitting a button to send a notification it gives following error.
502 - Web server received an invalid response while acting as a gateway or proxy server.

Is anybody knows the issue, Any reply will be highly appreciated.

What I have tried:

I have checked answers and blogs regarding same error message but unable to get exact result