Setting up Device Sandbox Test Environment

Eventually, you'll want to test that the products for your application are properly configured in App Store Connect, so that you can do final testing before App Store submission.

On device Sandbox testing offers similar benefits as Xcode based testing, so you can also still use Sandbox to test with your App Store Connect configured products.

Creating Sandbox Test User

Open App Store Connect
Go to Users and Access and click on the Sandbox -> Testers item on the left side.

Add the credentials for a new unique test user. Make sure to use an email that is different from the Apple ID you signed in with. Use a short, secure password that is easy to type in.
Run your app on a device in Sandbox mode. You will be prompted for a test account login. Enter the credentials of the sandbox user you just created. After that point, you can go into the iOS Settings app, choose App Store, and at the bottom of the screen will now be a "Sandbox Account" option.

Click your Sandbox account in iOS Settings. You will see all your Sandbox subscriptions as well as options to reset intro offer eligibility, or change subscriptions.