@Guyver13 

What are their operation modes of the 3 Decos? Are they all AP mode?

Do you mean all the 2 slave Decos connect to the main Deco by wireless? The Deco will select the Deco with better performance to connect, you can relocate the 2 slave Decos to give it a go.

With the 1st Deco connected to the main router only, please connect your computer or cellphone to the Deco's network and run the speed test for 3 times. Please show us the screenshot of the test results. Meanwhile, please click here  to check the wireless negotiation speed of your computer. 

Besides, please connect your computer to the main Deco by wired cable and check the wired speed you can get.

You'll have to explain what the modes are. I'm not up on the terminology. 

It's been set up straight out of the box following the app. 

This is how it's all set from my end of it makes sense

This is the speeds I'm getting while just the main Deco unit is connected. 

My Xbox one is struggling to stream anything on Netflix or Now TV. It's barely above 10mbps and it's only 20ft from the main Deco unit. 

Nothing in the house is finding a 5ghz signal. I could select it manually but my central heating thermostat will only work on the 2.4 frequency so I can't use that feature. 

My desktop is connecting at over 200mbps when plugged in directly. Also the older WiFi speeds are from the original router, which is miles faster.

@Guyver13 

So the main Deco works in router mode, and the speed with the main Deco connected seems normal compared with the bandwidth you paid. 

For the Xbox streaming, how far away between the Xbox and main Deco? Try to move it closer and you can add it into the high priority list and give it a go. 

You said all the devices use 2.4GHz, no one uses 5GHz, you can enable 5GHz and fast roaming, then the devices which support 5GHz can connect to it to get faster speed. 

With main Deco powered on only, what is the speed or signal strength you can get from computer room and attic.

And with all the 3 Decos powered up, you can check which Deco your device has connected, and then you can test the speed from the specific Deco. And then adjust their locations to get better wifi signal strength. 

You can run the speed test tool on the Deco app also. 

The Xbox is around 20ft max away from the main unit. 

Central heating will ONLY work on 2.4ghz frequency. 

Nothing in the house is connecting via 5ghz unless I switch it to only connect via 5ghz which then stops the central heating thermostat from working. 

All tech in the house is connecting to wrong units. 

Solar inverter in the attic is connecting to the living room. Battery inverter in the computer room connects to the living room even though it's less than 5ft away from the computer room Deco unit. Xbox which is in the living room only connects to the attic Deco. 

I've given up and it's being sent back to Amazon for a full refund. 

The reason I bought these were because the Virgin Media WiFi signal didn't work upstairs. 

But it does everywhere else in the house and gives a constant 200mbps. 

We also don't get any streaming issues. 

@Guyver13 

I see, totally understand your desicion; sorry for the trouble caused here, and thanks for your cooperation and effor to do the troubleshooting with us.

If need any further help in the future, do not hesitate to contact us. 

Have a nice day.