Everytime I go into Game Mode my UI disappears. The only things visible are the Game Name Text and the Background (on a 2560x1080 monitor). If I go into Game Mode on a 1920x1080 monitor I can see the Game Name Text, the Background and the Play, Loadout, Settings and Quit Buttons, and all children of the Play Pressed Gameobjects Gameobject.
If I build the project and run it the same happens.
Screenshots down below. Would be nice if someone could help 
Have you tried checking how far the “display” is regarding the camera. Try setting Canvas Render Mode to Screen Space - Camera, and attach your camera into the “Render Camera” box, and make sure that the plane distance isn’t too low or too high.