Hey this is what we did to get the soft shadows.
Select the Directional light and set the Mobility to “Static” (this means no distance field shadows) then in the Lightmass rollout there is a value for “Light Source Angle”. Higher values mean softer shadows. A value between 4 and 8 works very well to get this overcast look.
The Directional light brightness should be low and most of the light is coming from the Skylight. The Skylight mobility needs to be set to “Static” on mobile but on PC it can be “Stationary”.
Also make sure the lightmap resolution on you Staticmeshes is high enough.