To test in app purchases you need to create sandbox user. Go to App Store Connect and open "Users and Access", then – "Sandbox Testers":
You can sign in to Sandbox account separately from your App Store account since iOS 12. Sign in to sandbox here:
Purchasing process is the same as real in-app purchases. The only difference is accelerated time and impossibility to cancel subscription manually. Subscriptions in sandbox renew 6 times per day maximum.
Duration in sandbox
If after purchase in Apphud you can see a user with active subscription, it means that payment process work correctly.
You can test following events in sandbox:
Intro Expired (if payment mode is "Pay as you go" and number of introductory periods is at least six)