Oh yes, I see what you mean now, thank you for making the video!
Just place a rear sight to go with this front sight and it will work (the front sight alone won’t work without the rear sight):
For that location, you’ll need a canted rail for the rear sight too. You’ll need to make room by removing both the rear sight and the optic that are initially there . After placing the rear sight, you can place the optic back.
Regarding you not seeing the other player in multiplayer, check this setting before you hit play and make sure you are not choosing “Play Standalone”:
