AP in every test is TP-Link? If moving your AP 20m away gives you a hooping -40 in signal and better performance, it means there is either an alignment issue (i doubt it given your capabilities) or interference. Someone must have put something which greatly disturbs your signal path. I would recommend a 5GHz link if it is possible, or playing around with alignment, or some metal grids behind the antennas to see if they help isolating your signal path.
No, as I said the manufacturer of the AP doesn't make a difference, be it a TP-Link or Mikrotik. And you misunderstood that AP part, I set up another AP(Client side alignment was the same, the one I'm having issues with.). It worked fine, but doesn't work properly when connecting to devices over long range. See, I can easily fix it by switching to a 5GHz link but I want to find a solution first. It isn't like it was messed up from the start, it was working for a good part of two years so you can see where I'm coming from. Especially since I've access to most of the routers/CPEs in the neighborhood. Noise floor isn't too bad either, varies from -102 to -105, frequency channels are properly set up as well/interference can't be an issue. If it was a hardware fault, I don't think it would've worked with the "TEST AP" I set up. Now?