I have a map setup with an Environment Light rig, point/rect lights, etc.
I test this on both my laptop(RTX3080) and Geforce Now(Also RTX30xx)
In Fortnite graphic settings I have exactly the same settings on both (Epic).
Yet, as shown in the pictures below, the lightning on Geforce Now is way off.
Have you tried the obvious (although I’ve missed this before lol) - adjust the brightness of the MONITOR itself (also checking whether it has some sort of auto game mode setting on the actual monitor that increases brightness)?
Also, try another map just to see if it’s something odd about that particular one! :o)