To use a sandbox account (not your Apple ID), you must first add it to App Store Connect (https://appstoreconnect.apple.com/). Then do the following on your device: Go to Settings > App Store Enter the login-password for the account in the "Sandbox account" block Go to Settings > Apple ID Tap on "Media & Purchases" and tap on "Sign Out" After testing the subscription, you can cancel it by going back to Settings > App Store > Sandbox account > Manage. To interact with the App Store and Testflight services again, just go back to Settings > Apple ID and turn on "Media & Purchases" (tap "Continue" in the window).

iPhone XS iOS 16.5 (20F66) It works, but does not connect to the service the first time

iPhone XS, iOS 16.2: Still doesn't work. Resetting doesn't help.