GMS 2 Resizing background with steady rest [HTML5]


I want my game to look as if it's full screen on all major aspect ratios for mobile, so no black bars.

I tried the fullscreen scaling tutorial here:
But for some reason it isn't working at all?

At least I'd like the background to be full screen (with the corrrect aspect ratio)
And the other objects to render at a constant size (not scaling with the background).

Does anyone have any suggestions on how to achieve this?