Apple doesn't skip a client app because it needs to be able to pay, but how?

Question

Apple Developer Program

I have a website where people can buy access to some kind of learning system. If you have paid for access, you can download the application for Windows or Android (from Google Play), enter your username / password and safely use what you paid for. You can just download the application from Google Play, but then you will just have demo access.
Decided to make an application for iOS here. I did it, but Apple tells me that two requirements are not met:
a) there is no registration in the application itself (that's okay)
b) there is no payment for access within the application
And I don't understand the second point. The general essence is clear, Apple does not like that the loot goes past him, but the application is just a client. You have already paid somewhere, so you use the application as an additional feature, and not the main one. It is so?
Can you tell me how to play it? I'm not talking about how to cheat, but to decide. Simply, if the client has already paid on the site, then he should not pay in the application. And this cannot be if I subscribe to Apple's requirements.