Your Instantgo subscription is billed through iTunes.
Your Instantgo subscription auto-renews each month until you cancel. The cancellation steps are the following:
Cancel your subscription billed through iTunes
IPHONE, IPAD, OR IPOD TOUCH
Go to Settings > [your name] > iTunes & App Store.
Tap your Apple ID along the top of the screen.
Tap View Apple ID and sign in if prompted.
Find and tap your Instantgo subscription. Important If you don't see an Instantgo subscription, make sure that you're signed in with the same Apple ID that purchased your subscription. Also, review your iTunes purchase history. To find out how, go to See your iTunes purchase history.
Choose Cancel Subscription (iOS 10 or later) or turn off Automatic Renewal.
For more help, go to Manage your subscriptions billed through Apple