This may be old but hopefully this helps someone.
The answer you need is that you need to add a setting the defaultengine.ini file.
[MobileSlateUI]
bTouchFallbackToMouse=false
This will go inside of the DefaultEngine.ini file that is located in /Config/DefaultEngine.ini
But this seems to only work for me when I am either in a packaged build or a standalone game not sure where I need to put it yet so the games run in the editor also work.