You need to update to the latest Samsung Gear OS before you can connect to the iOS app. The only way to do this right now is to first pair to an Android phone and do the upgrade there and then reset and re-pair with an iPhone. It works great after that but you will only be able to install the free apps from Samsung and the free watch faces.
I know Samsung mentioned they would be rolling out the ability to use the S2 w/iOS. I haven't seen anything else since CES. Anyone see any updates since? Loving my Note 5, but have a 6+ as a backup and wouldn't mind playing around with the combo.