Deco m9 Plus solid red light
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I purchased the 3 unit Deco M9 Plus system in December 2020 and it has been awesome, until recently when it has started failing, with all three units showing a solid red light.
My network uses a BT Smart Hub as a modem (only), connected to the main Deco unit via an ethernet cable. The 2nd ethernet port is cabled to a wall socket connected to a 16 port switch, which then serves the whole house with a gigabit network. The two other Deco units are also wired to wall sockets, serviced by the 16 port switch, so the mesh setup has a wired gigabit backhaul. When working correctly, the Deco app shows the following:
The Operation Mode is Wi-Fi Router, so the Decos are issuing IP addresses to connected devices, while the BT Smart Hub simply provides internet access only.
My broadband service provider is BT and I get a reasonable 76mps download speed, which is adequate for most things, including working from home.
A couple of weeks ago I lost internet access and the Deco App showed the internet connection had been lost. All three Deco units were showing a solid red light. Checking the BT Smart Hub, it still showed the blue light, indicating it still had an internet connection. I connected a laptop directly to the BT Smart Hub and it worked fine i.e. internet access was available! After trying to reboot the deco units and the BT Smart Hub and switch numerous times, eventually, the Deco app reported the internet was available, however the two other deco units (Kitchen and Office) were shown as connected by wifi and some devices on the network, my main desktop PC included, still couldn't get a network connection. After even more hassle and time wasting, it eventually came back up and everything looked okay. It wasted about an hour of my day! When the issue initially occurred, the Deco app reported the following:
The other day, the same thing happened - no internet connection reported by the Deco app, solid red lights on all deco units, but the BT Smart Hub still showed it had an internet connection! Rebooting the Deco units didn't fix the issue, so after Googling and finding this forum post, which appears to be my exact issue, I turned off the main Deco unit (Loft), rebooted the BT Smart Modem and waited until it had an internet connection (blue light displayed), then turned back on the main Deco unit, which then showed a green light. The other two Deco units were still showing red lights and clearly weren't going to "fix" themselves, so I rebooted each one in turn by disconnecting it from the power and waiting for the green light. Eventually, all Deco units were showing green, with a wired backhaul. 1 hour later, the same thing happened again while watching Netflix!!!! I couldn't be bothered, so went to bed and sorted it in the morning.
Having got it up and working again I was "lucky" to have a full Friday without issues and contacted BT who sent an Openreach engineer out on Saturday. They tested the internect connection from the BT master socket and everything was rock solid! The history showed I had good, consistent download speeds and there were no signs of issues. Interestingly, when they plugged the BT Smart Hub back into the BT Master Socket, once the modem had got the internet connection again, everything worked as normal i.e. I didn't have to intervene to reboot the Deco units!
So, the problems I'm encountering are being caused by either the BT Smart Modem or the Deco units. I will be asking BT to supply the new Digital Voice version of the BT Smart Hub as we will soon be switching over to digit so hopefully they won't object to providing this newer hub a little earlier. I don't believe the BT Smart Modem is the issue, but this will rule it out completely.
It has now been nearly 3 days since the last issue occurred, but having had 3 events like this in the last 2 weeks, I no longer have any faith in the Deco units and as I'm an IT contractor working from home, I can't afford to be wasting time "fixing" whatever issue this is, if it occurs during the day.
TP-Link Support, have you any idea what might have started causing this issue with the Deco units, given they have worked flawlessly for 2 years and 8 months?
@David-TP Thanks for responding to my post.
When I was gathering info for the post, I did notice the firmware version was out of date so have updated to the latest version. Obviously, given the Deco units have been working perfectly for 2 years and 8 months, I don't believe the firmware is the cause of the issue, but it's always good to be on the latest version, providing that doesn't break anything 
I prefer the Deco to assign IP addresses so I'm reluctant to change the config to Access Point and for the BT Smart Hub to act as the router. Out of interest, why do you think that would "improve" the situation?
If the issue occurs again I will switch the Deco's around and use a different one as the Main Deco as you suggest.
Fingers crossed I haven't had another issue since Friday 18th am, so that's 3 full days now.
Regards.
Paul.