hey there I was just searching for this answer myself! At first I was sure it must be some setting in the “Exponential Height Fog” actor but no.
Turns out the console command you are looking for is:
r.HeterogeneousVolumes.MaxTraceDistance [number]
Not sure what the default is (it’s more than 5000) but it’s too short for real world clouds 
I set it to 50000 and that fixed my problem.
Thanks to this post: 5.3 Heterogeneous Volume Rendering tips and tricks