When you sign up for the free trial you would have had to press a button notifying you that you would be charged for an annual subscription after 30 days.

Unfortunately, as with any other apps in the App Store and Google Play, all of the billing and the framework used for app free trials and subscriptions is controlled by Apple through the iTunes system for iOS devices and Google through Google Play for Android devices. You would need to contact Apple if you have an Apple device or Google if you have an Android device and raise a case with them if you think you are eligible for a refund.