@Andrea_LaPaz 

As long as you plan to have that as a single Deco WiFi mesh, with M9-1 as Main Deco, it will work.

See this document for helpful info: General questions about Ethernet Backhaul feature on your Deco

It is recommended the switch to be gigabit unmanaged. TP-Link brand preferred. In my house, I use TRENDnet TEG-S80g 8-port and TP-Link TL-SG1005D 5-port switches with Deco.

On a side note: have you bought M9s already or just considering them? These are capable units and I went with M9s for my home, but for me important was form factor (needed unit I can wall mount) and tri-band for wireless connection between Deco units.

If form factor is not important for you, and because you plan to wire all Deco units, consider Deco X20. About the same price as M9, but supports WiFi6. That will future proof your WiFi, especially if your Internet connection is gigabit or you expect it to be in the not so far future.