@Alriyada 

As far as speed you will get the best possible speed having the Decos wired. In you case if you need more coverage, but you are restricted by the cable, you can then disconnect the cable and have them communicate via Wi-FI. The Deco M9s have a dedicated wireless backhaul so you should not see any dramatic differences. 

It might be best to place them where  you get the best coverage, then test the speed as you might be perfectly fine with them talking over Wi-Fi and being spread out more.

thanks for your reply . What is the maximum number of devices that I can add because the current 3 devices are not enough to cover the entire house and I think of adding 3 more pieces. There is another question. The main objective that was purchased because of the Deco is to ensure that the Internet does not cut when moving from the upper floor to the lower floor , especially to make a call over the Internet Is it really Deco not disconnect and move from one device to another is done smoothly?

@Alriyada 

The seemless hop from one node to another is possible by the protocols 802.11k, 802.11v, and 802.11r.

What ever device that is being used would need to support those protocols or else there will be no seemless connection. It would be similar to having a router that is MU-MIMO, but if the computer you are using does not support it, will have no way of fully utilizing MU-MIMO.

You can have up to ten Deco M9 nodes so you should be fine with six around your residence.

@Tony

This week will receive my another 3 pack system I'll back to you after install it .

I enable QOS In main router can you let me know how to disable it ? becasue when I select Standrad QOS I can't disable maximum upload /download value 

@Alriyada 

Here is a link to all our support offices. Please contact your nearest one for additional techincal asisstance: https://www.tp-link.com/en/about-us/contact/

With respect to QoS, leaving it on Standard is the normal setting were it is not "enabled". The speed can be edited manually or automatically by using the speed test.

@Alriyada Deco M9+ is pure junk that tp-link refuses to support!   They know about the issues and are covering them up!   That is what CHINA does - they cover stuff up!!!