@David-TP hi. I have same issue with m4r which fails and the other 4 m5 works fine 

  @David-TP Thanks for your reply.

Here is my network topology. As you can see I'm a loyal Tp-Link customer :). Everything is connected through the ethernet backhaul. I am not leveraging the second ethernet port on Deco's.

The Deco M5 #1 is the main unit. What I can do is to move it to be directly connected to Archer router instead of M4 for testing purposes, but I would prefer to have faster WiFi as they are in the current configuration.

The Deco's work well for 1 or 2 months, after that period I start having the red blinking light in both M4 units. The M4 are still providing wireless access but no internet connectivity - seems to me they aren't able to reach the internal network. I am unsure of the triggering factor of the red blinking light, this might be a time issue (after X hours), or a spike/sink from the electric company, I don't know. But this is happening with that frequency. My father in his house has 2x M4 and never experienced any issue with them, so seems like it has something to do between M4 and M5. Again, hard rebooting M4 brings back to normal operation.

I think I can give a try to the beta firmware as you suggested in another reply, even though my Deco's are connected through ethernet. I don't know if this might help in my case but I can give it a try

  @David-TP Everything is working fine so far. I've updated just one of the two M4 to verify if it will improve over the old one. I've had the last red light blinking event last week, so I guess it will take some 4+ weeks to have again the same issue.

If the 1.7.0 is going to fix that, I guess I'll have the red blinking light only on the other M4 which I didn't update. Glad that the updated M4 now has the IoT wireless network.

I'll keep you updated in this thread if something good or bad happens.

Thanks for the help so far.

  @David-TP brief update, today the Deco M4R I had updated to 1.7.0 beta firmware was blinking the red led.

The other M4R with the latest stable release wasn't affected and was still working.

I had to hard reboot the faulty M4R by pulling the plug to have it back online, so seems that the beta firmware hasn't fixed this issue.

  @David-TP Happened again today, both the M4R Deco became suddenly offline - red led blinking. Had to restart them.

Unfortunately, the cables are in the wall - cable between Archer and TL-SG108 #1 and #2. Archer and M4 #1 are 1 meter away, Archer has no wifi enabled.

Cables have no more than 2 years and sustain the whole wired traffic and they work well. The whole wired clients are attached to TL-SG108 #1, #2 and #3. In my house there are like 20 IoTs, both wired and wireless, up at any time and never had issues. More than 5 PCs are powered on/off, not counting mobile phones/tablets.

I have updated to 1.7.0 just the M4 #1. M4 #2 is at the latest stable, automatically updating. Doesn't seem the 1.7.0 fixed the red blinking issue.

The two M4 are very far and their wireless network won't overlap. In my father's house, he's using just two M4R in AP mode and they work flawlessly. I again suspect the issue is mixing M5 and M4 in the same network. Never happened on M5 to become disconnected - M5 #3 is very far and cable runs underground on the exterior of the building and never had any issue with that, M5 #1 and #2 partially overlap the wireless network, again never had any issue with them.

Maybe it has to do with some power shortages, power holes in the grid, transients, but the rest of my equipment didn't reboot and stayed up and running, so it would sound very strange to me.

  @David-TP Happened again today. One of the M4R was shut-off and powered two days ago as I had to move it to a slightly different place in the same room. Nevertheless, the two M4R were put offline at the same time.

I am suspecting an issue in the M5 main unit (AP mode) while dealing with M4R(s). The other M5 units were still working as usual. At least this M4R which has been rebooted is within reach of wireless connection to the main unit, although they are connected through cable. In any case, when M4Rs are blinking red, there is no way to let them reconnect to the main unit unless they are rebooted by pulling the plug and reconnecting it.

Everything has been updated to latest available firmware, no changes.