@RoArJr 

I'll start with basics. Deco mesh provides its own WiFi network. You should turn off WiFi in your ISP router, because if it is on that could cause WiFi interference and other issues between two different WiFi networks. If you use same SSID for router WiFi and for Deco mesh, it is even worse.

With WiFi in router turned off, you should have the following network topology:

ISP provided FIBR ROUTER (no wifi) > Main Mesh node M5 (wifi5) > mesh nodes M5 (wifi5)

With this, if you wish to upgrade to X60s, the answer to your question is very much obvious: you do not need to install a wifi6 capable router. You will keep your FIBR router with WiFi turned off, and replace M5 with X60.

This will be your network topology after replacement:

ISP provided FIBR ROUTER (no wifi) > Main Mesh node X60 (wifi6) > mesh nodes X60 (wifi6)

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Hint: if you have settings and customizations in your current Deco mesh you want to seamlessly transfer to new X60s, it is possible to do that. Make one new X60 join your mesh network. Promote it to Main Node. Add rest of X60s to your mesh. Delete all M5s from your mesh. Done. (use Deco app to add, promote and remove Deco units).

@Alexandre. 
 

thank you very much for the simole explanation and help!

cheers!