If you can't connect your Google Home to the Deco M9 system, then this is the solution which worked for me:

Reset your Google Home (long press the sound button on the back of the device) until it says "Resetting".  Then in the  Google Home app click the + symbol and set it up as a new device.  Same thing for the Google Home-Mini (there is a very small circle button on the bottom of the Mini to be pressed to reset).