@lainelaz 

This will work (in fact, that is what I have with my Deco mesh), but you must configure Deco mesh to run in Access Point mode. Check these two TP-Link FAQ articles:

1. General questions about Ethernet Backhaul feature on your Deco

2. How to set up Deco to work in Access Point mode

  @Alexandre. 

Thanks for the reply!

If i understand correctly, i need to run access point configuration only for the second setup:

router>ethernet cable>deco3 

or for both setups?

Thanks again for your time

  @lainelaz 

In Deco mesh, Router/Access Point mode applies to all its Deco units.

If you have Deco mesh with three Deco units configured this way:

router>ethernet cable>deco1>wireless>deco2

router>ethernet cable>deco3

it is single setup as far as Deco mesh is concerned and it will only work with Deco mesh running in Access Point mode.

So i kinda have problems seting up this way:

router>ethernet cable>deco1>wireless>deco2

router>ethernet cable>deco3

my deco 1 is my main, then i add deco 2 through setup by pressing add deco unit.

but when i try to add deco 3 which is connected directly to router (like deco1) then everything becomes unstable, i start to loose my internet randomly and the app is getting offline randomly

deco mesh is running in AP mode

or should i create a different network for the deco3 ? 

Thanks!!

  @lainelaz 

Do not create different network for Deco 3: you will lose WiFi mesh benefits and will end with two separate independent WiFi networks.

Is Deco 3 works OK if it is wirelessly connected to the Main Deco? This would be the first thing to confirm, just to be sure it is not hardware issues with Deco 3 unit.

The Deco Ethernet backhaul feature is based on the standard IEEE 1905.1 protocol. Assuming everything is right with Ethernet cables, your router might be the rare case of not supporting that standard. What is the router brand name and model number?

Check router's specs for supported protocols or contact ISP Tech support. If your ISP offers different routers, ask to replace yours with different brand/

If nothing else works, the only other option I can think of is to connect Deco 3 by Ethernet cable to Deco1, instead of router.

  @Alexandre. 

Thank you so much for your reply.

My router model is Technicolor DGA0122 which i believe is good for such configuration...

I wonder if my set up was correct so i will try to explain my steps what i did in first place..

So first i plugged deco 1 to my router and it became main deco, then i have changed my network settings to AP mode,

then via app i have added second deco2 wirelessly and did the same with deco3, 

so my configuration was like this (AP mode)

router>Ethernet cable>deco1>wireless>deco2>wireless>deco 3 (it works flawlessly) 

after that i just plugged my second ethernet cable from my router to deco 3, and i thought it will automatically  change the configuration to this :

router>Ethernet cable>deco1>wireless>deco2

router>Ethernet cable>deco3

But what happens is the app starts to go nuts, it only sees main deco or main deco and another deco which is  named "custom" ??  and no internet...

so maybe i have did something wrong during these steps?

  @lainelaz 

I have similar setup and everything works just fine with Deco in Access Point mode. It is possible your ISP router does not support Deco backhaul protocol.

Run the following test. Bring all three Deco units to where Technicolor DGA0122 is, and using short Ethernet cables of good quality wire them all to separate Ethernet ports of router:

router>Ethernet cable>deco1

router>Ethernet cable>deco2

router>Ethernet cable>deco3

Power on Main Deco and wait till it is online. After that, power on both Satellite Deco. If you've got unstable Deco mesh, that strongly points at incompatible router.

Then, you may wish to talk to ISP Tech support and have them confirm if Technicolor DGA0122 does not support standard IEEE 1905.1 protocol, if that support can be enabled in router, or if it can be replaced by a different router that does support that protocol.