@tpliiiink 

I have run tests with wired Decos in AP mode, and I am doubtful all traffic goes through Main Deco. I tested on very similar to your network layout. If you want, I can explain in more details test I ran and why I think so, and how you can easily verify it yourself.

Yet, there is another simple solution and I wonder why are you not considering it: buy new  X20 and deploy it in house A as a Satellite Deco for your Deco mesh. After succesful addition, make it Main Deco. Now you have Main Deco in house A.

You can set Main Deco in Deco app. Touch globe on the top of the main screen, you'll see list of all Decos you have, then touch three dots at the right upper corner and menu will appear:

@Alexandre. 
@tplinksupport

So I bought a new set of Deco X20 with Black Friday and I haven't been able to set them up.
as quick summary I have 2 houses on same property all wired up and I initially set up house B with X20 late august successfully. Now I have added house A with 3 more nodes (6 in total) and having issues. Everything is through Ethernet backhaul

Now when a new client connects, even if hard wired the Deco seems to assign the address himself by DHCP instead of my main modem (that acts as router/dhcp). When it does this my modem freezes and only reset works (Freebox Revolution)

One thing to mention is that the new sets of x20 are hw v1.0 and my old one (bought in august) we're v2.0. I ve tried thought the app to change the main deco 

anyone can assist? Otherwise I will return them

@tpliiiink 

Have you added new X20s to existent Deco mesh network, or have you created 2nd Mesh network for them? If you added them to existent network, you should see all of them together in Deco app:

I have four, but you should have six listed there. 

@Alexandre. 

Yes I had all the 6 under the app. The issues I have is sometimes the deco assigns the IP instead of my router. This creates confusion and my router hangs.

Deco's set as access point

@tpliiiink 

Usually you can have Deco in Access Point mode assigning IP addresses when it loses connection to router, Freebox in your case. When connection is restored, Deco will yield to the router and everything is supposed to recover on its own.

Deco may have exposed an underlying problem with your wired network. I am not sure why adding additional X20s triggered it, but as long as Main Deco is connected to Freebox, Main Deco should not have started to behave as DHCP server.

Also, your Freebox router has a problem: it should not just hang because there is another DHCP server briefly appeared on the network. 

It is not possible to turn that Deco feature off yet, that will come in future firmware. I may be able to offer advise how to mitigate the issue, but troubleshooting will need your time. If you are not interested, or have very short return policy on new X20s set, you can just return these new X20s. I can't guarantee success, so don't keep them beyong end of return date.

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If you would like to troubleshoot this, please do the following:

Check if your new X20s are on the same firmware version as old X20s. If not, it would make sense to upgrade them to latest firmware.

Let me know how far your Main Deco is from Freenet router (are they in different buildings?) and what is between them on wired network: switches, what brands and models. 

Please share screenshot from your Deco app of the same page with Deco units I shared, and also from Advanced page, this one: