If you previously purchased the Premium, you can unlock the features by restoring the purchases you made in the app. This can also allow you to use the Premium Pack on a 2nd device using the same iTunes account.
Note: If you originally invited 7 friends, restoring purchases will not unlock the Premium. You can either re-invite another 7 friends or purchase the Premium.
When purchasing the Premium, make sure that you received a confirmation at the end of purchase. It should look like this:
Here's how to Restore Purchases:
In the app open the Settings>Purchases>Restore Purchases!
This will restore purchases made with this iTunes account. (If you purchased the Premium using another account, you'll need to log into the other account in order to restore).
If restoring purchases does not work, we suggest to "re-purchase" the feature. Apple does not bill twice for the same product - so you if you purchased the Premium in this iTunes account, you can tap the "buy" button again without being charged. After pressing "Buy", Apple will show the message "you have already purchased this product" in a pop up (instead of the regular pricing / "buy product" pop-up). These pop ups are controlled and managed by Apple and the iOS (not by Sync.ME).