@BasK 

It is most likely the cable that Deco units don't like for any reason, but perhaps there might be a workaround which could make it work.

If you connect one side of that cable to Main Deco, and another side to laptop, will laptop have Ethernet link?

  @Alexandre. 

Hi and thanks for the reply! Unfortunately the lights won't blink in that configuration either. 

I won't be able to pull a new cable via the wall. They must have spilled some concrete during construction.. whatever has happened there. 

Think I'm better off to put a switch at the location where I wanted to place the ethernet-based slave Deco unit and connect the clients (e.g. Ultra HD TV) to the switch. Presuming the switch *will* accept this cable. 

I have a LAN-cable tester yet it needs a 9V battery which I don't usually have in stock. Will buy and post about the results. 

  @BasK 

That was going to be my next recommendation: using the switch.

Also, question for you: do you have ISP modem or ISP modem/router? Because, if it is router, you could configure Deco mesh to run in Access Point mode and, then, won't need to link Main Deco with Satellite Deco. In that configuration, you can connect Satellite Deco with Ethernet cable (through switch) directly to the router. 

Main Deco can be connected to the different Ethernet port of ISP router.

See How to set up Deco to work in Access Point mode

  @Alexandre. 

It is for sure a modem/router 'all-in-one' solution for people with need-for-broadband/TV/telephone.

Can it be a combination? Because: slave unit #2 needs to have wifi to the Main unit (ethernet not possible), but slave unit #1 can go - via Ethernet - directly into the ISP modem/router. 

Will Mesh be function in between the 3 units? 

  @BasK 

Then, you should configure Deco mesh to run in Access Point mode and everything should work, wired and wireless, assuming you can manage to make that Ethernet cable work through the switch or otherwise.

Here is my home setup, with M9 Plus and M5 Deco units, in Access Point mode.

Main Deco - Ethernet wired through switch connected to the router

Satellite Deco #1 - wirelessly connected to Main Deco

Satellite Deco #2 - Ethernet wired through same switch as Main Deco

Satellite Deco #3 - Ethernet wired directly to the router

The only caveat is to have switches compatible with Deco Ethernet backhaul. Avoid D-Link brand. TP-Link brand is a safe bet. More about it here: General questions about Ethernet Backhaul feature on your Deco

  @Alexandre. 

I've also tried to connect the main Deco unit via the 'suspected' cable directly to the modem/router: doesn't work. 

How will the switch - in between the modem/router and the Deco unit - then make a difference? 

  @BasK 

That would be the plan: connect one side of that cable directly to modem/router and another side to the switch. If switch can negotiate connection with the router, you can connect anything you want to the switch and it'll work, including Deco unit.

I would recommend to look for a switch that has LED lights on its ports reporting status. Just having "Link on/off" LED may not be enough. Find a switch that can report speed of connection on each of its ports, too. Even if it is a bit more expensive.That option may become handy in troubleshooting if switch decided to negotiate speed with the router less than gigabit. 

Here is an example of such switch:

Unless, of course, your ISP router does report this info, then you can just check it on a router side. Example: