Use the sample code from the DevTouchMovementMode documentation page: StarterPlayer.DevTouchMovementMode
It does what the documentation suggests.
It throws an error:
Insufficent permissions to set DevTouchMovementMode - Studio
The current identity (2) cannot setDevTouchMovementMode (lacking permission 5) - Client - LocalScript:4
Not sure. Trying to find a way to disable the mobile UI temporarily and this is one of that solutions that probably would have worked if not for the error.
Issue Area: Engine
Issue Type: Other