@David-TP 

Thank you very much for your quick response.

If I use the XE75 in AP mode as you suggest, will it keep its functionality of "mesh" so I can move all around my house without loosing signals even by changing AP's connection ?

In my new place I have RJ45 cat 6 running from the floor 0 to each floor. Do I need an unmanaged Switch right after my modem/router on level 0 to increase the power of the signal to each floor or I don't need one ?

  @DamienLM 

In AP mode, Deco will keep its mesh functionality. You should be able to walk around your house without losing signal, with wireless devices seamlessly roaming between Deco units.

Of course, that is only possible if you have WiFi signal overlap between different Deco units. If there is a dead zone where WiFi signal does not reach, when you walk through it you will lose WiFi connection, obviously.

I also recommend to have fourth X75 on Floor 0, you can place it by the Main Router.

It is better to connect Deco units by Ethernet cable. If you have enough Ethernet ports on your Main Router, and cables reach there, you may not need the switch: just plug cables to the router and connect Deco units to the other end of these cables. 

Deco unit on Floor 0 can also be connected directly to the router, or to switch.

Deco X75 has three Ethernet ports. If you don't have enough ports on Main Router for all Deco units and will add X75 on Floor 0, you can connect X75 on Floor 0 to Main Router and use other two X75 Ethernet ports to connect two X75s to them. In that setup you will only need 2 free Ethernet ports on Main Router to connect four Deco X75 units.

  @David-TP 

Hi David.

I'm getting 8gb Fibre optic in a couple of weeks and would like to make best use of the 2.5gb ports on my Xe75 pro. 
 

I'm looking to get a similar setup as the OP so my questions for you (or anyone that can help) are:

-can I use the QNAP QSW-2104-2S 2ports 10GbE SFP+ 5port? If not, which one do you recommend?

-will the Ethernet backhaul work?

just to confirm, my setup be: ISP router ->switch 10gb->wired Deco in AP mode. 

thanking anyone who will help ❤️

  Up please. 
I hope someone will be able to help with my questions above. I didn't want to create another thread just for it. 

  @Ordinadim1 

-can I use the QNAP QSW-2104-2S 2ports 10GbE SFP+ 5port? If not, which one do you recommend?

Yes you can

-will the Ethernet backhaul work?

If you connect the deco's together via ethernet, yes

just to confirm, my setup be: ISP router ->switch 10gb->wired Deco in AP mode. 
Yes

  @wesselkoning 

A million thank yous for your response! 
 

so if Ethernet backhaul works when the switch is behind the modem with all decos connected to the switch then I'll do that!