On a new device, or after reinstalling, open Inkline settings and tap Restore Purchases. Because subscriptions belong to your Apple Account rather than to the app, signing in with the same account is all that is required.
If restore fails, confirm the device is signed into the Apple Account that made the purchase, then try again after a few minutes. App Store receipts occasionally lag a reinstall.
Still stuck? Back to the help centre or use the contact route there.