I have been a happy customer with a 3 node Deco M5 setup for a few years now. Howver, in the last few months, I have had issues where the deco system ends up in a bad state where some units can connect to the wireless network but no network traffic is forwarded. I typically notice this with an iPhone, and at first it almost always works to either enable or disable Private Address for the unit, which gives the unit a new MAC address. After a while though, this trick stops working. The laptop or iPad right next to the troubled phone typically works without problems, at least for a while. Then progressively, less and less units works. I have found that restarting the Main deco addresses this issue quite effectively, and everyhing works as it should for maybe a day or so, however, it seems like number of functioning hours that I get from a deco restart is decreasing after each incremental restart, and after a while it only helps for a handful of minutes. At this point I have found that disconnecting all the decos from power for a bit brings back the system to a good state and everything typically works for weeks after this.

I hope the above description is useful to people that might encounter similar problems. I also have a few theories about what might be going on. A few months ago iOS 14 was released. This household has many iOS units. One of the features of that OS is to use Private Address, which means that the phone (or iPad) will use a randomised MAC address that will change on each connect instead of using the same one on every connect. The fact that switching this feature on or off and when doing that getting a new MAC address resolves the issue initially when it pops up makes me think that the Deco firmware keeps track of seen mac addresses in memory in some way and that when the system (the 3 decos working in tandem) have used up the allocated memory for these mac addresses things start to break down. It also seems likely that the different units forwards their lists of seen mac addresses to each other in some way, such that the three units all need to be turned off for the list of MAC addresses to be completely emptied out.

That was a bit of speculation from me, I would be very happy to hear anyone else chime in with thought or ideas, and I hope that tp-link resolves this issue in an upcoming firmware version.