Widget UI issues in "Standalone Game/Mobile Preview" mode

Hello Everyone,

I’m a beginner in the Unreal Engine universe.
I have a strange issue related to “Standalone Game/Mobile Preview”.
Whenever I start the game with one of these modes (or package the game and run it on Mobile) I just get a dark screen without any UI/HUD.
However, if I run it in “Selected Viewport” or “New Editor Window” then it runs perfect.
You can see here my simple blue print for this level. I have the feeling I have something not configured correctly or missing in GameMode or something, or maybe the Widget itself is missing something.

The result of the mainmenu UI if I run this level in Selected Viewport" or “New Editor Window” :

Now if I run it in “Standalone Game/Mobile Preview” or any product enviroment then I will just get the following:

Now if I switch to my first level, and Play in “Selected Viewport” or “New Editor Window”, same thing it looks perfect :

Same as Mainmenu, if I run this level in “Standalone Game/Mobile Preview” or any product enviroment I will have a strange camera angle and no UI/HUD:

If you need any further info about my Blue Prints please let me know.
I’ll be very happy to know the reason behind this behaviour and teh difference between the Modes “Selected Viewport” or “New Editor Window” and “Standalone Game/Mobile Preview”.

Thank you !