@MarekDE 

What you see is by design. Deco does not extend wireless network, even in Access Point mode. It runs its own WiFi network. 

If you can position two Deco nodes you have to cover all building, that's great. If not, you will need to buy one more for optimum coverage.

  @Alexandre. According to the official statements X90 could operate in AP mode to extend existing WiFi thru the ethernet cable connected to the Deco X90 unit.

  @MarekDE 

Pehaps, this will help: TP-Link OneMesh™ VS Deco Mesh: What's the Difference?

TP-Link Archer ax11000 is OneMesh.

Deco X90s are Deco mesh.

The last paragraph of the technologies description from the link I provided:

In summary, though Deco Mesh and OneMesh™ support Mesh networking, they are totally different technologies used by different TP-Link products, so they cannot build a mesh network together or with other brands. Choose one of them based on your actual needs and start building a seamless whole home mesh network for your home today.

  @Alexandre. I think that this is clear statement.

"Access Point mode aims to extend the existing wired network and expand the wireless coverage for a home network. At this time, the Deco system will not create its own subnet but purely forward data between the root router and the clients."

When the TP link will solve that problem? I see that US version had different firmware then EU so does their version work?

  @MarekDE 

I saw this document. I am not sure if it is intentionally written in such a way, or just happened to be confusing. I am going to dissect for you what it says and what it does not say.

It says: "extend the existing wired network"

Does not say: "extend the existing wireless network"

It says: "expand the wireless coverage for a home network"

Does not say: "expand the wireless coverage provided by the router"

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There is nothing to fix by TP-Link, what you see is by design. For your house, to have Deco WiFi mesh everywhere you will need three-pack Deco set, not necessarily X90s. Based on what you said about your house, 3-pack Deco X60 can do just fine, and at least where I live 3-pack X60 is cheaper than 2-pack X90. You may save money if you can return X90s and buy X60s.

  @Alexandre. How much will TP-Link pay you to get away critism by turning my request for help with deco x90 in AP mode into explanation how to read text in english.

AP is access point and doesn't matter which brand of router you are using problem is that X90 offers AP mode but this isn't true.

Next time please Tp-link instead of using "AP" in their X90 decriptions use the correct name because otherwise it's confused or its cheating on customers for which router in AP mode means the same.

  @David-TP 

Thank you for your feedback but I do still have a lot of questions.

Why there are two non-compatible apps Deco and Tether and both are doing almost the same? 

Why it's impossible to connect Archer AX with 2x Deco X90 as master and slave mode?

Why there is non-compability between them? It's some kind of company politics?

  @David-TP I am new to this, and a lot of above goes over my head, I have a two pack X90 and want to connect fist one to existing router in AP mode, which all I have read seems to say I can do. The problem is the App downloaded for this does not seem to have the "more" or any other buttons to enter the X90 to select AP mode, so my question is I hope simple. How do I put and X90 onto access point mode?

thanks