I recently installed three M9 Plus units to replace my Virgin "Smart"hub and had a seemless start. I have a number of Hue Lightbulbs and had the Hue Bridge plugged into the main Deco unit. Due to only having two ports on the unit, I needed to use one for the Virgin ethernet connection, so I moved the Hue Bridge onto another Deco unit upstairs.

Since doing this, both the Hue app and Apple Homekit app (which I mostly use to control the lights) have struggled to find the bridge and bulbs. It will eventually find them but it can take a number of minutes or need the wifi turning on and off. 

Any ideas? Weirdly it seems to be continuning to work fine on my wife's iphone

Cheers