@Tony 

I have done what you suggested, I am still facing the same problem of no connectivitiy on my computer. However, I have noticed a new behavior. On the device list, my device is shown as offline, but on my computer I am shown that I am connected and there is "No Internet."

I have read threads on Killer's website that their hardware seems to have problems with TP-Link routers and there is a custom fiirmware fix that TP-Link provides. Does something like this exist for the Deco M9? For more information on my computer's wifi chip, I am running the Killer 1550i (Intel 9560). My laptop is able to connect to all other networks, including my mobile hotspots, perfectly fine. It seems to be an isolsted ongoing and frustrating issue with the Deco M9.

@srihou I do. As soon as I bought it, I was having issues and tried to get it replaced. TPLink and the third party retailer just played throwing hats at each other. A $300 mistake i made.

Anyone want to buy my Deco M9 plus mesh system ?  If they cant or wont issue a firmware update, this disconnect issue must be hardware related and can not be fixed!!! 

@srihou I have just purchased the M9+ and have been trying to set up all day and night and have just given up. Half of my devices have connected and are ok, some connected and now won't reconnect and the others just will not connect... pulling my hair out... well what's left of it. Any advice would be appreciated. 

@srihou I got the same problem but I finally found a solution!

I was operating a router from the Internet provider in front of my Deco router who also broadcasted a WLAN signal. This signal seemed to cause the connection drops.

I simply deactivated this second WLAN source and it works perfectly now since one week.