On the Premium page, you can now also remove the detected in app purchase from within the app. From Settings, tap the image at the op of the screen. This shows you the Premium screen and from there you can remove the detected in-app purchase.
Note, this does nothing on Apple or Google’s servers where the purchase is registered. This is merely to allow you retry a Restore of the purchase.
After the removal, restart the app and it should be running in non-premium mode. The menu shows “Upgrade to Premium” as for any standard mode user.
Head into the Premium screen via the menu button and tap Restore.
If you still have problems, see How to send app log file to support