@G_Stoica 

It appears you did all the right testing. There is not much else that can be suggested, except maybe one more test to make absolutely sure Kitchen Deco hardware is OK.

Bring Kitchen Deco to where Hallway Deco is and connect it to Hallway Deco with short Ethernet cable. See if Kitchen Deco can establish Ethernet backhaul.

If it does, that would mean XE75s do not like your 38m Ethernet cable. I am not saying cable is bad as you have tested it, but it is obvious XE75s can't use it.

At that point, the only options are to either return XE75 and try different brand of WiFi mesh, or try workarounds. Perhaps, try workarounds and then return XE75 if nothing helps.

So, do this test I suggested and if you would like to discuss workarounds, reply here. For workarounds I'll need you to have one or two network switches you can spare for testing and/or network coupler.

  @Alexandre. thank you very much for your answer!

This weekend i had some time to make some tests:

1. Tried all Deco's ethernet ports and all are working (connecting cable directly to laptop and running a speedtest).

2. I took Kitchen Deco up and wired with a 12 meters cable to the Hallway (Main) Deco and ethernet backhaul was established, wired laptop to Kitchen and run a speedtest, the result was around 900Mbps.

I bought a TP-Link LS1005G switch and i plan to add it in the middle of 38m cable, i'm thinking it will provide enough power in order to have the Hallway and Kitchen Deco wired properly for backhaul.

Otherwise i'm very pleased with XE75 and i plan to keep them because no other solution managed to work in the entire house until now. It works like it is in the screenshot but speeds are around 100-300Mpbs stable on the ground floor and i just want to boost it up a little with wired connection :)

I will keep you posted with the result, i also bought another long cable for testing to check where "it breaks" the connection, probably will help others to know exactly how long you can go, in my mind everything under 50m was good to go but it seems not.

Hello!

After i tried with a 50m cable in order to test how much length the cable must have i immediately realized that it works with 50m cable. Then i started worrying about my installed cable, took the cheapest cable tester on the market and... surprise, pin no. 2 was not connected even on the bigger tester was showing that is ok. I just added new sockets at both ends and now everything is working as desired. 
 

I don't know if the first tester failed at that point or crimping tool is not that good and cable was able to move inside after i tested but for sure you can run a 50m Cat 6A between two Deco's.

Now the speed of the network is between 500-700Mbps all over the house, tested with Wi-fi 6 wireless card which is more than enough for me.

Have a good day!