@michalOl 

For this to work, Deco mesh must run in Access Point mode. See Switching a Deco to Access Point (AP) Mode

After a discussion with phone support, I have removed the TP Link Router from my setup and the first Deco is now the receiver of internet connection. I changed all cables for CAT6, yet the other 2 units still connect to the main one via wifi and not ethernet.

  Hello @David-TP,

I do have a TP Link switch in the 'server room', so it's:

ISP provider -> CAT6 from optical converter (sever room) -> CAT6 in the wall -> main deco (living room) -> CAT6 in the wall -> switch (server room) -> CAT6 -> CAT6 in the wall -> satellite deco (1 in bedroom, 1 in study room).

I think that the problem is whatever goes from the main deco. Because wifi speed of the main deco is pefrect (I pay for 500 MB/s and I can measure up to 500 MB/s. But if I connect my laptop with CAT6 cable to my switch, the measured speed is only around 100 MB/s.

So I believe that my satellite decos prefer wifi because it is actually faster than the cables (which I have no idea why is that).