If it doesn’t work, is there any error message? If Apple rejects you, there should be errors or exceptions when the program is running, or you can publish it to the App Store and try again to see if you can receive it, because we don’t know much about the situation. So I can’t give a very specific answer

You can refer to my article about playing with APP payment and using HTTP2 to implement Apple push, both of which are made using the micro-framework of Flask.