Why is wifi not connected in one corner of the room with a direct line of sight to the router?

Question

WiFi

Lord, good day. I am writing here out of desperation. The problem arose somewhere half a year ago and still can not solve it.
There is a room 8x8 meters. There is a wifi router hanging on one of the walls.
In one corner, on the heel of 3x3 meters by the window, devices tightly refuse to connect to wifi.
The density is naturally very high. WiFi Analyzer and similar software does not show any anomaly. The signal throughout the room is stable and high. The number of visible APs does not change.
The symptoms are quite the same: connection attempt, dumbness for N seconds, connection error.
Venda, android - everything is one. Changing the channel does not solve the problem. Channel width 20 \ 40 does not play a difference.
Unfortunately, there is no 5 ghz AP to check, and there are almost no such clients.
Replacing the hardware didn't help. Setting a point in this room did not give any result either - before it calmly punched the signal through the walls.
What could it be? A bad neighbor with a microwave, a bank on the ground floor?
Maybe some devices? Or what profile of the company can be contacted?
Or maybe it’s not at all desirable to be in this room?)
UPD: I changed the old horned dot to ubiquiti unifi ap, hung it on the ceiling in the center of the room, the result is unchanged.

Answer 1

photo of the studio

Answer 2

Behind the wall is a microwave oven or a neighbor's router

Answer 3

Have you solved the problem?