I’m really too new and unfamiliar with Unreal so most of the time I use the method (totally wrong) “trials and errors” :o …because there are tons of parameters that, even if displayed and adjustable, they effect the scene only if another option is enabled.
I’ts so confusing ( don’t know how so many people that are new with unrealengine can produce such photoreal scenes 
…maybe in a couple of years I wil be able too 
If you want you can send me your scene so I can try to make some illumination test on it, then I can send it back (if I reach a nice result) with all the parameter setted.
Cheers,