My home network has five TP-Link Deco: three of them are M5 while the other two are M4. The M4 are V2. FIrmware version is the latest for any Deco.

All of them are connected through ethernet cable. One of the M5 is the principal device. The Deco network is configured in AP Mode, as I have a separate VDSL router which manages DHCP, firewall and internet routing. All of these devices are connected to the same ethernet network.

The issue I have is after a while (about 1 or 2 months), both the M4 start blinking red light, approximately at the same time. They are unable to connect to M5 master for some reasons. When this happens, all of the M5 Deco's are fine and any wireless client connected to the M5s works just fine. The red blinking Deco M4 are still able to accept wireless clients, but then the clients cannot reach the internet. 

I have already ruled out any hardware connectivity issue, seems like the M4s are not able to connect after a while to the M5 and the only thing I can do is to power them off and back on to regain the M4 connectivity. The Deco are "near" each other to be able to connect wirelessly if for some reason the ethernet cable fails, but not very near (5+ meters and some tick walls in between).

I have all of them since 3+ years and they always showed the same issue, no matter the firmware updates in the past years. This blinking only happens on M4 Deco, not M5 which are always functional. My father home has 2x Deco M4 with similar setup and he never experiences the same blinking red light, so seems like the issue is only happening when used in conjunction with M5.