On the other side of the pitch-black room there's a router, which must be restarted. There's a catch - an enemy patrolling the area. Fortunately for you, the enemy is blind as a mole, especially because it's so dark in here, but you can't do two things while moving around the area - don't use flashlight and don't sprint. Move while crouched, hiding in the darkness, or behind a cover when the enemy is nearby. Remember to not stand on the monster's path - it will immediately notice you and attack.
When you reach the console responsible for restarting the equipment, press the yellow button and flip the lever located near it. Afterwards, immediately hide in the shadows, as the enemy will be drawn to your position. Don't move away too far, as you now have about half a minute to flip the lever again. Wait for the enemy to move away, do the activity and hide in the shadows again, as the monster will return to check what is making a noise.