@Yibar 

Satellite Deco X60 can connect to Main Deco M9 and they can run together. Your WiFi capable mobile device will be able to connect to Deco X60 using WiFi6 protocol. Do not expect WiFi6 speeds in this setup: Satellite Deco X60 will deliver at most half of Internet speed you are getting from Main Deco M9.

If you want to keep M9 as Main Deco, you might get faster speeds from having Satellite M9 as well, instead of X60. You can also consider other way around: a pair of X60 (Main and Satellite) might get you better Internet speeds than M9 and X60.

  @Alexandre. 

Thank you for the reply.

I will connect the x60 with ethernet backhaul to my main Deco m9 which is connected to the modem with ethernet (all are cat 6a). Will the wi-fi 6 speed still won't be optimal?

  @Yibar 

If you plan to use Ethernet backhaul, X60 speed will be in the WiFi 6 range, and with gigabit Internet connection you should expect X60 WiFi speeds to be in the range of 800Mbps-1Gbps. 

You didn't mention Ethernet backhaul in your original post, I assumed Satellite X60 will be connected to Main M9 wirelessly. Ethernet connection changes everything (for better).