Recently I've been having an issue where certain devices can't reconnect after going offline (ie if I turn off the computer) and less commonly being completely dropped. I've been using the deco m9 for over a year with no issues previously. 
 

The only way I can fix it when this happens is a reboot of the main deco unit. Rebooting the other units don't help. There was no commonality to the clients being affected, it varies from iPhones to windows laptops to a chrome book (pixel slate), though some devices never seemed to be affected (ie a smart TV and Apple TV). All affected devices were used on my desk which is just a couple of feet away from a deco unit. More recently, it seemed to have improved with my windows laptop more frequently reconnecting, and only very rarely dropping on my iPhone. However my chrome book can never reconnect in the morning, and always require a reboot of the main deco upon which the chrome book can immediately reconnect. 
 

Also interesting is I noticed my windows laptop lost its connection to the deco. When it reconnected, the laptop reported getting an ipv6 address but limited connectivity. That said, the ipv6 setting is off on the deco and I had checked immediately after seeing that message. Not sure if that's part of the issue or just a coincidence/issue with the laptop. 
 

Lastly, I thought I observed this as the private wifi address feature rolled out on iOS. However I turned it off on all my devices and the problem persists. Ive also tried turning off fast roaming which didn't help.