We have a Deco M9 Plus (version 2) that we purchased from Costco (and which replaced an aging, but still functioning, Asus RT-AC68u), and which worked fine for about a week after running whatever stock firmware came with the device.
However, starting yesterday (June 10, 2021), and about a week after upgrading the firmware to 1.5.1 Build 20210126 Rel. 59387, I started seeing strange behavior.
First, certain devices would show as being connected to the main Deco, but would have no internet connectivity, while other devices connected to the internet normally. If I attempted to disconnect one of the non-connecting devices from, and then reconnect to, the wifi network, the device would either not reconnect, i.e., it would attempt to reconnect, but would ultimately fail, showing a globe icon rather than a wifi icon (a Lenovo Thinkpad laptop), or I would get an invalid password error (an iPhone 12).
A reboot of the client device (the Thinkpad or the iPhone) would not resolve the issue , although a reboot of the mesh system (whether a reboot of the main node, or of the entire mesh system) would resolve the issue.
However, this happened throughout the day, and it culminated with the entire network going down - devices would either be connected to the wifi signal, but would not get an internet signal, or the devices could not connect to the wifi network at all. Again, I did a full reboot of the mesh network. This time, I unplugged the Decos, waited about a minute, and then plugged them back in. This again resolved the issue.
Throughout the day, I checked the modem, but it showed a solid signal, although I do note that, of the 16 downstream channels on our modem, 10 channels have a power level slightly greater than +7dB (range of 7.1 to 7.7). Our internet provider (Cox) was not reporting any outages.
As far as the mesh itself, the main Deco is connected to the modem (SB6183) on the first floor, the second Deco was also on the first floor, but on the opposite corner of the house, and the third deco was upstairs in a central bedroom (house is about 2100 sq. ft.). In all three cases, the Deco was on top of a shelf, with no obstructions. Also, the Decos were connected to each other via wireless backhaul, as we live in an older home that is not wired with ethernet ports in each room.
The only changes to the mesh I made were:
(1) I set the LAN IP to 192.168.1.1 (from the default of 192.168.68.1, or whatever the default was)
(2) I set static IP addresses to 4 devices (a wireless printer and 3 IP cameras)
(3) because of the invalid password error, I tried setting the Primary DNS to 22.214.171.124 and the Secondary DNS to 126.96.36.199. (for IPv4), which was a solution I saw mentioned on the TP-Link forums
(4) I did port forwarding for the 3 IP cameras
(5) I reserved a TP-Link DDNS
(6) As noted, I updated the firmware about a week ago
As a note, I also set static IP addresses on our old Asus RT-AC68u (running two using AIMesh), and I did the same port forwarding rules, yet we were not experiencing the same kinds of dropouts we were experiencing yesterday with the Deco. I did have to run two routers in a mesh, as one of the rooms upstairs received a very weak signal without a second mesh node.
I also thought I had too many Decos, which might be causing some sort of unexplained interference, so I tried removing the second Deco that was on the first floor (so I only had two Deco's - one of the first floor and one on the second floor).
However, when we woke up this morning (6/11/21), none of the devices we tried (several iOS devices and an Amazon Echo Dot) had an internet signal, even though the devices were allegedly connected to wifi. Again, rebooting the entire mesh system resolved the issue.
P.S. - although my devices could not connect to the wifi network, I was able to connect to the Deco remotely. Specifically, if I disconnected my phone from the wifi, and just used a cellular signal, I was able to connect to the Deco via the Deco application, which showed the Deco as being online, which is how I was able to reboot the mesh system.