@Gwenn 

You need to have one of the Deco units wired to the modem/router, the other two will connect wirelessly. What would be recommended is to disable the modem/router Wi-Fi so there is not any interference with the Deco.

@Tony 

I successfully installed all Deco units with one as the router as I disabled the Motorola router.  It was difficult to do as the Deco app kept asking for the wifi password and when we entered the correct one, then we got a message that it was an incorrect password.  Quite frustrating.  Now we have an issue connecting to our existing Sonos speakers and sub.  I connected one speaker to the Deco router by ethernet cable and the white light flashed on the speaker and then the light went out and no light was there.  Do you have any suggestions on the Sonos connections?

@Gwenn 

The Sonos gets confused since the 2.4GHz and 5GHz broadcast the same name.

I believe some users mentioned they wired their Sonos to the router, set up, then they were able to connect it via Wi-Fi.