> or only allow to buy the point ( coin ) via In-App Purchase?
Yes, that. Must. Only. No doubt. Just do it*.
Unless you've got some otherwise edge-case scheme in mind that you're for some reason keeping to yourself, of course - in which case I'm almost sure there are maybe alternative solutions that can be perhaps be wedge-fitted into something you'd only know if it might have a chance of success by running it by App Review in real time.
Just remember that Apples' pretty well seen it all by now, so if you decide to run the review gauntlet and DIY, bring your (service) game, not your name 😉.
*Quoting the ASRGs:
3.1.1 In-App Purchase:
If you want to unlock features or functionality within your app, (by way of example: subscriptions, in-game currencies, game levels, access to premium content, or unlocking a full version), you must use in-app purchase. Apps may not use their own mechanisms to unlock content or functionality, such as license keys, augmented reality markers, QR codes, etc. Apps and their metadata may not include buttons, external links, or other calls to action that direct customers to purchasing mechanisms other than in-app purchase.