These days it’s fairly inexpensive to have depth based per pixel fog. You can make a custom node and put it in all your materials while adjusting distance, density and color from blueprints. It can also be applied as a post effect. You can even add calculations for height or color depending on the sun’s screen space location to have really ugly low life atmospheric scattering thingy 
BTW, I thought unreal fog defaults to something on mobile. It doesn’t sound right not to have it already.