Help needed: subscription structure inside iPad App

In 2016 we developed an App for recording video, storing the video in a separate compartment (so it is not visible in the camera roll) and on demand uploading the video to our secure video hosting portal.

At that time getting it to work functionally was prime key for our project, we did not yet see possibilities to build in a subscription model.

We now want to build in the possibility to have a subscription to uploading to our servers. This annual subscription (and monthly, if possible) should renew each year. When the subscription finishes, people may still use the App for recording, but uploading should be impossible.

We are looking especially for example projects with a subscription model. There are a lot of examples and tutorials for IAP apps, but almost all of them focus on a single In App Purchase.

Technical details: the app is iPad only, iOS 8+ at the moment, built in Objective-C and for instance using NMSSH as cocoapod to connect to our server, The app is distributed through the VPP for Business program to customers in The Netherlands and the United States.

We are looking for directions how to build in such a subscription model. Who can help on this?