Are you getting the disconnection message in game or on the mobile app? Can you specify which iPad you are using to connect the second screen? The issue derive from the functionality of second screen isn't fully available yet. Are you getting an error message when attempting to access it?
I am using an iPad 2 I think, one of the slightly older ones at this stage.
I dont get any error msg until it looses connection, and that comes up on my iPad, the bottom right hand side of the screen. It's not because my screen locks either as I already have it set to never lock unless I manually do it
Let me know if there is anything else you need to know, and hopefully you can help me out.