SimpleMeasure can also be activated by an in-app subscription. Depending on the operating system, an account for the respective app store is required (Android: Google Play, iOS: Apple App Store). In order for the subscription to be verified, you must be logged in on the device with the account with which the purchase was made. If there is an active subscription, the app is automatically unlocked when it is started. Note that completed in-app subscriptions can only be managed by the user (see links to help articles below).
[iOS] Active subscription is not detected automatically
On iOS, it can happen that an active subscription is not automatically detected, especially after updates. In this case, first make sure that the subscription is actually still active and that you are logged in on the device with the correct account. If this is the case, you can restore the subscription via the settings (cogwheel button at the top right) → "Restore purchase".
General help articles on managing in-app subscriptions
iOS: https://support.apple.com/en-euro/HT202039
Google (Android): https://support.google.com/googleplay/answer/7018481