This method uses materials on cylinders with UW plane mapping, not fog.

This is our basic terrain and gradient sky we are starting with:

Within our terrain plane, and the sky dome, are 2 cylinders with reversed normals and caps removed.

The inner cylinder has what is basically our terrain material [a mix map] fading to transparency by a gradient ramp in the opacity slot.

By itself it looks like this ... already this lends a sense of greater space:

Now we apply the material to the 2^nd, larger cylinder ... this starts with 100% self illuminate ... turn off gloss while you are at it ...

So far we have this:

Add some volume fog gizmos if you want to “cheat”