I found four of those so far, all on the east side of the island and facing NE, if I recall. They don't seem to have any traverse mechanism, which makes me wonder if they're some sort of electromagnetic orbital launch system. Either that or really long range artillery.
Have you seen them open up? As I've seen them with the sides of them open up as if either powering up or something like drones fly out of them but not too sure though.
The simple answer is they are the launchers for the drones if you notice the compartments or flaps on the top of it could be compartments for launching the small drone swarms