First of all, I'd like to let you know that Apple does not allow iOS devices to remotely controlled, and so you can only share the screen of such devices over a Teamviewer connection. That means that you can see what the iOS device holder doing and only that.
But in order to have the screen transmitted over a connection, you must first set up the screen sharing to the iOS device. To do that you can follow the steps described at this related post and after that, you will be able to connect flawlessly.
I hope to have made it clear why are you may be facing such issues with iOS devices.