@LeSade 

I am using TP-Link Deco mesh in AP mode.

I have three Deco connected directly to ISP Router: one M9 Plus and two M5.

I am running one of Deco M5 through MoCA adapter. Getting excellent speeds: MoCA adapter behaves just like gigabit Ethernet cable.

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Some ISP routers may not support Deco Ethernet backhaul protocols. These are minority, but they do exist.

Some MoCA adapters may not support Deco Ethernet backhaul protocols. You haven't provided MoCA adapter brand name and model number. Mine are ScreenBeam Bonded MoCA 2.0 Network Adapter (Model: ECB6200S02), they are compatible with Deco.

Depending on type of Internet connection, your router may be MoCA capable. You haven't provided type of Internet connection you have, also ISP Router brand name and model number.

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The first test I would recommend you to do is connect both Deco M5 to ISP Router with short Ethernet cables. If Satellite M5 reports Ethernet connection to ISP Router, router is compatible with Deco mesh backhaul.  

While on that configuration, run speed test through Satellite Deco to verify it delivers good speeds. It should deliver same speeds as Main Deco if both are wired to ISP Router and it supports Deco mesh Ethernet backhaul.

  @Alexandre. 

Did you setup this from the begin? I saw in the app it says to create router mode first, than add decos and the last step is to change it to AP mode.

can you tell me what i need to do? maybe i need to add one by one?

  @LeSade 

You can change Deco mesh Operating Mode from Router to AP and back any time. Switching a Deco to Access Point (AP) Mode

It is easy to find if your Deco mesh really runs in AP mode: tons of features will disappear from Deco app, such as parental controls, antivirus, connection type.

Also, for the reference, when Satellite M5 uses Ethernet backhaul you should see the following in Deco app for that Satellite Deco:

Same for MoCA adapter - if Deco can use MoCA adapter to establish wired link, it'll report that link the same way, as Ethernet, in Deco app. When you see WiFi backhaul, that means M5 can't use Ethernet cable or MoCA for backhaul, even if wired.