First, I’d open those curtains. Leave the window unobstructed and keep the tree right in front.

How are you watering? What are you using to tell it’s time to water? The brown is usually a sign of over watering or salt buildups in the soil from not flushing.

Based on your reply, overwatering is the likely cause of the brown spots.

Don’t water on a set schedule. Water as the plant needs it.

Sharpen a wooden dowel and stick it in the soil all the way to the bottom (this will not harm the roots) and pull it out and inspect.

Dark, damp with soil stuck to it = don’t water.

Clean, and dry = water.

Dont water the leaves, just the soil. When you water, do so until you see water coming out the drain holes, but down ever allow the plant to sit in that water.

Again, I’d open the curtains during daylight hours.

It may be root rot or another issue. This post may be helpful for you in figuring out what is causing the brown spots and how to successfully treat your Fiddle Leaf Fig Tree. How to Treat Brown Spots on Fiddle Leaf Fig Leaves