I'm trying to sign into a Sandbox account which is visible in App Store Connect from an iPhone 6 running iOS 12.5.5. After submitting Sandbox account or Apple ID credentials, a gear wheel appears but when it disappears the Sign In prompt reappears. If I enter invalid credentials, immediate feedback they they are invalid is shown.
Cant sign in to App Store Sandbox account
After adding a subscription from a new Sandbox account (set in App Store Connect) the iPhone shows the email address of the new account as Apple ID: newemail, but clicking on only shows Cancel and Sign In options, not Manage.