As far as I understood, I have to use my own server to verify subscriptions. There is no other way directly from the app, which I would prefer. (I tried to convince Apple for my solution using consumable products on App-Review, but they refused it.)
It seems Apple gives no other opportunity to implement subscriptions. I tried to avoid that, because I have no experience building an own server. But it is better than throwing away the whole project.
So yes, you would do me a big favour, to show me how to implement subscriptions, I mean how I validate them and connect to apple.
I am sure this could be interesting for other users as well in future.
Thank you