In your app, don't put a link or any text that would lead or let user know they can make payment outside of App Store. Set up an external website that register

In your app, don't put a link or any text that would lead or let user know they can make payment outside of App Store. Set up an external website that registers/login user and process user payment there. Then in your app, require user login and then your app can communicate with the backend to check if the user is subscribed.