In-app purchase or third party payment vendor for app subscription model for a physical device?
late but may be useful for new people


for unlocking features/content you shall use in-app purchase
for products/consumables (that not implies the previous item) you can use external payments like Stripe
if you want to avoid in-app purchase, you can do the netflix way, give the app without register/guest, just login with an account previously purchased in the web or android device. But for this your app should have a known brand, otherwise no one will go to your web, make a purchase, come back to your app and login.


source: https://developer.apple.com/app-store/review/guidelines/#payments