@Hulao
One more thing I will need from you, but you can put it in reply after proceeding with steps I provide below.
I'll need to you tell me what Main Deco is connected to: modem, router, or gateway - and also its brand and model number. That may come handy if initial troubleshooting fails to find the cause.
Also, please turn off MAC address randomization on iPhone you use for speed tests, and you only need to do that for Deco SSID.
Now, as promised.
1. Not all Deco units use Ethernet backhaul, even when wired.
In Deco app, open page for each Satellite Deco and see what it says under Signal Source. If Satellite Deco uses Ethernet for backhaul, you'll see this (as an example):
In my example Living Room is Main Deco, in your Deco app you'll see different name but the important thing is Ethernet link.
If there is any Satellite Deco that does not report Ethernet - that will narrow the problem. If all report Ethernet, continue with next step.
2. For some reason, when you test speed on some of Deco units your test device (iPhone) is connected over 2.4GHz instead of 5GHz.
Turn off 2.4GHz on Main Deco network, under More/Wi-Fi in Deco app:
Re-run speed tests on smartphone, and also double check it is indeed connected to the Deco for which you run speed test. If every one reports 400Mbps, we are done. If not, continue.
3. Network switch is incompatible with Deco mesh backhaul.
Unplug from the switch Ethernet cable that comes from Main Deco. Plug that cable to one side of network coupler. Unplug from the switch Ethernet cable that comes from Billiard Room Satellite Deco. Plug that cable to another side of network coupler.
Power off Satellite Decos still connected to the switch: Office Room, Kids Bedroom, Main bedroom. We won't need those and powered off they won't interfere with the rest of tests.
You now bypassed networking switch and have daisy chain similar to this:
More specifically:
Main Deco<-Ethernet->Billiard Room<-Ethernet->Living Room<-Ethernet->Kitchen
Verify that all three Satellite Deco in that chain report Ethernet in their Signal Source, starting with Billiard Room. That to ensure network coupler is not defective and every Deco is going through wired connection. Just to be sure.
Run speed tests from iPhone for all these three Satellite Deco. Do you have 400Mbps on all? If yes, the problem is networking switch, if not - that is inconclusive and please continue.
4. Some of CAT6 Ethernet cables are not up to specs and Deco units can only negotiate 100Mbps over those that aren't.
Plug Windows computer to the available Ethernet port of Kitchen Deco and run speed test on wired link. Have you got 400Mbps from speed test on wired connection?
If not, keep tracing through that daisy chain by unplugging Satellite Decos and plugging Windows computer. Your Billiard Room delivers 400Mbps, so when you end with Windows computer connected to it you should see 400Mbps. If still not, go to Main Deco and plug Windows computer with short good quality Ethernet cable to the port which used to connect Main Deco with the switch. For sure you must get 400Mbps from Main Deco when Windows computer is connected to it directly. Let me know.
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When you provide me with summary of results for above steps, I'll see what else can be done if cause is still not clear.