I've pretty much givin up on my Deco M9 Plus at this point. Talked to support a couple times and have only gotten generic responses that haven't helped. Figured I'd throw a last ditch post on here before I order another mesh solution from a different brand.
Deco M9 Plus has worked perfect for about 3 months after getting it in August 2020. In December 2020, started having wifi drop issues. Did a bunch of troubleshooting and found out it was the designated "main" deco that would drop wifi, but any device connected to the other 2 satellite deco's would still remain connected. Also, anything hardwired to the "main" deco still remains connected. Power cycling the "main" Deco is the only thing that resolves the issue. When the issue first started, it was maybe once a day. Now it can be multiple times a day, very frequently.
What I have tried:
1. Setting Deco DNS servers to Google's 18.104.22.168 rather than using my ISP's (recommended by support). No change.
2. Reset Deco's using button on bottom. No change.
3. Played with some settings like disabling Beamforming and fast roaming. No change.
4. Split up the 2.4GHz network and 5GHz by enabling the Guest network. I found this recommendation based on some googling I did and other user's having similar issues. It was actually posted in a knowledge article on TP-link's website as a "fix" but it's really more of a workaround in my opinion. I THOUGHT this actually worked, as the issue seemed to stop for about a week or so. But it eventually returned.
5. I thought maybe it was hardware related to the Deco that I was using as the "main" one. I swapped one of the satellite's over and set that one as the new "main" Deco through the app. No change. New "main" Deco still drops wifi.
6. Used the manual firmware upgrade tool to upgrade to latest firmware. No change.
I don't really have anything left to try without basically disabling functionality of the product itself, or trying to use only 1 Deco instead of the 3 that I have. Ready to purchase new product as I work remotely from my home and I can't deal with ongoing reliability issues.