Instant Notification Settings Not working


I have implemented the onetime payment button in my application based on the code from the following URL,”.
Later, I have hosted the application in a public IP and port 443. I have done debugging on the published application and it worked. Except, I am not getting any results from the Merchant URL(method name IPNHandler) which is set on the Instant Notification Settings. Please see the attached image.
Does anyone know why is this? Please share details.