For what I know, there should be some scene settings met to be able to achieve "God Rays" effect, or at least make them visible.
The scene should be in the shadows, so dark or relatively dark. Look at the references you will probably see almost all of them have a dark image (in shadow cast from other objects), where rays are distinguished (going through trees or small window).
That is the thing, if you work with clients immediately mos of them would say "It's too dark, keep the rays, but make it bright", which you cant.
So you need contrasty dark scene with a bright light (usually sun), then you can add the volumetric.
For Corona I would advise not to use the Global volume material for this case, as the volume material would need to be dense to achieve that contrast we need. And on global volume it affects the lighting too, as the sun gets dispersed in the fog.
You need to create a box for every camera and give it a volume material, but leave camera out of the box, so it does not affect it. Play with the settings like in the tutorial burning offered, also do not forget to turn on single bounce only as that makes it a little more contrast in volume. That is it probably.
To the devs: thinking now about this, is it possible in the future to add a distance from camera in which the fog starts, like a minimum value similar to clipping.
