Hi !
Make sure you're logged out from your apple ID on the top of the settings page and from the App Store > Sandbox Account. See below :
Now when you try to buy a product or a subscription in your application, you will have a pop-up to log in, use the credentials created for your sandbox tester. See below :
Now you'll be logged in in Settings > App Store > Sandbox Account and you'll be able to manage your subscriptions.
Topic:
App & System Services
SubTopic:
StoreKit
Tags: