maybe you can try this tutorial,it work for me:
Creating a photorealistic rain scene in Unreal Engine lumen tutorial
Cinematic Fog BP:
If you have problems using it for versions under 5.1. Try packing those cVars into a blueprint.
r.VolumetricFog.TemporalReprojection 1
r.VolumetricFog.GridSizeZ 256
r.VolumetricFog.Jitter 1
r.VolumetricFog.GridPixelSize 4
r.VolumetricFog.HistoryWeight 0