I'm getting multiple times rejections for the same reason:
Regarding 3.1.1, the subscription can be purchased in the app using payment mechanisms other than in-app purchase.
To resolve this issue, it would be appropriate to subscriptions included in or accessed by your app must be available for purchase using in-app purchase.
My app is providing subscription-based video content exclusively in Tunisia (already indicated in the App review process), this region does not support IAP (based on this https://support.apple.com/en-us/102775), It's not possible to make IAP work consequently and I'm forced to provide local payment methods through a webview, this is still rejected for the same reason: make it IAP compliant ..
I've also provided similar app in the same region providing alternative payments and not implementing IAP and these app are in the app store already.