@leizerar 

In Access Point mode, you can connect Satellite Deco directly to the router with Ethernet cable. For that to work router must support Deco Ethernet backhaul, but most routers do.

  @Alexandre. 

Thanks !

But when I did it, and looked at the network diagram shown on the tp-link app, it looks like only the main deco is connected with a cable to the router, and the other 2 satellite units are interconnected wirelessly. How can I be sure that the satellite units are receiving the data via the cable and not via WiFi ? 

  @leizerar 

For what it worth, I have two Satellite M5s connected with Ethernet cables directly to ISP router, both report Ethernet link and have been like that for 2 years already.

I can think of three reasons why Satellite Deco can't use Ethernet backhaul when connected directly to ISP router:

1. Deco mesh runs in Router mode;

2. Ethernet cable issues;

3. ISP router does not support Deco Ethernet backhaul protocol.

I'll leave for you to double check that your Deco mesh runs in Access Point mode. Not only it should report that in Operation Mode, but you will have way less configuration options in Deco app, for example: no IP address reservation, no QoS, etc., etc. Full list of what you shouldn't see in Deco app when Deco mesh runs in AP mode can be found here: What’s the difference between Access Point mode and Router mode on the Deco?

The other two can be tested and if it is router that does not support Deco Ethernet backhaul, you may wish to contact your ISP to find if their router supports the standard IEEE 1905.1 protocol. If it does not and protocol can't be enabled, perhaps they have different router model they can swap yours with. Many ISPs offer more than one router model.

The topic of Ethernet backhaul and why it might not work discussed here: General questions about Ethernet Backhaul feature on your Deco

To pinpoint where the problem is, power off Satellite Deco and wait till it is reported offline in Deco app. Bring that Satellite Deco to router and connect it to router with short cable of good quality, preferably not more than 1 meter (3 feet) long. Wait till its LED is green and check its status in Deco app.

If Deco app reports WiFi backhaul for that Satellite Deco, let me know and I'll tell you what the next step is.

--------------------

For the reference, this is how my three Satellites are reported in Deco app: Bedroom is on WiFi as it should be, Office and Study that are wired directly to router - on Ethernet.

  @Alexandre. 

Hi,

I checked the AC mode and fiber as you suggested, its OK. Than, I did some research to find the spec. of the router on the Web. Its HT-360AX from Heights-t (you can find it on the web) GPON standard is IEEE 802.3bz 2.5G, which I assumed is different form the  IEEE 1905.1, but maybe I'm misunderstanding. So eventually, I extended the fiber to a new location (putting some sweat on it :) ), and placed the router in a new location in a way that allows me to connect the main Deco to the router, and the satellite Deco with the main Deco with Ethernet (The third Deco is connected wirelessly anyway).

Now I see in the app the satellite Deco connected Via Ethernet to the main.

One last question I do have - The app doesn't show the type of connection between the router and the main Deco, just shows "Main". How can verify that the it uses the Ethernet cable and not WiFi ?

Thank you so much for your support  !

  @leizerar 

Glad to hear you found a solution that works for you.

Main Deco requires Ethernet link to modem/router. It will not connect to modem/router by WiFi. Because of that, it does not make sense to show type of connection between the router and the Main Deco in Deco app: it will always be Ethernet.