First I would try the app without launching the overlay to be sure the activity scene is working correctly.
If it does turn out to be a conflict between the 2 scenes, check in the scene properties of the overlay and be sure it is set to overlay there.
> Thank for the answer Rich,
> I've tried to create the app with only the Activity and with only the Overlay... oddly, it shows the overlay fine but it doesn't show the Activity. It doesn't throw any error either.
Are you exporting a project?
Is the activity scene in the project?