not an expert myself, but you could try the following:
-add a skylight, that will brighten you’re scene a bit, just don’t over do it.
-another idea is to add an iterior light or just tweak lights, maybe add a more to the indirect lighting value
-you could use the postprocessing volume and play with auto exposure brightness
-if the window is not recocnized you might want to increase the nr of photons emited.(not likely to be the problem here)
not sure if there is a way to light the scene using just a light source, and hdri doesn’t work that great in unreal. so my be is add some lights 
take a look here: