@NikSf153 

Start with this TP-Link FAQ: General questions about Ethernet Backhaul feature on your Deco

If you have follow up questrions or it is not helpful, feel free to ask.

  @Alexandre. Thanks. I have read this thread. Still can’t figure. When I connected the main deco to the secondary deco directly; the ethernet backhaul showed up but when I connect the secondary deco to the poe port via switch; the backhaul happened through wifi again. What do I do now?

  @NikSf153 

Please, clarify for me, is this how your Main and Satellite Deco connect to the switch:

  @Alexandre. This is what my network looks like.

  @NikSf153 

Below is the picture from the TP-Link document I recommended you. See the last diagram on it, which is crossed out:

  @NikSf153 

You will need to connect them like it is on first or second diagram, with Main Deco connecting directly to ONU and switch or Satellite Deco - to second port of Main Deco.

  @NikSf153 

Depends, what is your ONU: device with just one Ethernet port, or is it Gateway (modem/router) type of device? If it is Gateway, you can switch Deco mesh to Access Point mode, but if it is not - your mesh will go offline when switching to AP mode and you may need to reset it.

Can you check your ONU type?

If ONU is basic type, you could get inexpensive WiFi router, connect it like on last diagram and then switch Deco mesh to AP mode. Turn off router's WiFi. This does not have to be advanced router, just capable of doing basic networking stuff. You are not going to use its WiFi, so it could be even old 2.4GHz only router - these should be very cheap if found online or in computer liquidation store. Definitely less expensive than X50-PoE.