Welcome to the world of (Mikrotik) wi-fi mysteries.
A cap Ax is designed to be mounted on a ceiling (though it is also possible to mount it vertically on a wall, though in this latter case it will likely perform worse).
The board is full of reports about the cap Ax being either very good or a terrible device, so we don’t really know.
There some settings/tweaks that may better the reception or speed, but not AFAIK a canonical guide on what should be done.
Take your time looking for related threads, here is a couple of the most recent ones:
http://forum.mikrotik.com/t/cap-ax-gen-6-mediocre-performance/176254/1
http://forum.mikrotik.com/t/cap-ax-performance-vs-cap-ac/177572/1