@moo_tada It is possible that 3rd Deco is too far from Main Deco to connect to, or your 3rd Deco experienced hardware issue. If you bring your 2nd Deco to that same place on a different floor, where...

@Smejkal Second option will not work with Deco in Router mode. Check this TP-Link FAQ: General questions about Ethernet Backhaul feature on your Deco

@Golfetta Different M4 hardware versions is the problem. TP-Link only has M4 V4 firmware that allows to configure how Deco connect to each other. When you added M4 V2 to your mesh, not only they do...

@gareth820 ... and one more thing: if you reconfigure your Deco mesh in Access Point mode, you'll have options on how to more evenly distribute clients between Deco nodes. I can discuss these options...

@gareth820 You have invested a lot into Deco mesh system, it would be waste if you just replace it. Also, with the new WiFi mesh you may have same issue. I have suggestions for you to try, perhaps...

Aidy2020 wrote @Alexandre. Thanks again, I tried this but I have quite a few products that don't work on 5GHz. Specifically Nest doorbell and smoke detectors and a few other things so that...