HDRI-Sky in AR-App?

Hi! I am using ARCore from google for my AR-Project and so far everything works well.

Client now wants a more realistic look and I wanted to use a cubemap on a sky. Problem now is, that when I put the sky in the Picture Frame Blueprint where my code and my objects are, there is nothing happening ingame. So in the picture frame itself immediatly it looks better as it looks like it should in the viewport of my BP. But when I play the game and hold the camera onto the marker the AR-Object just looks like it is default lit.
I tested with sky on stationary and movable.

I also done a test placing the sky in the level that is played later on, AugmentedImages. But again with no effect. What do I do wrong? I don’t find a solution no matter what I test.