@Alexandre. In the end it was indeed the cable. Even though it was working at 1gbls for every other device, the deco couldn't negotiate at 1gbps on it. Went out and got a new high quality Cat5e cable...

@Alexandre. Okay I tried your suggestion and something really weird is going on. See image below. In this setup, cable 1 is connecting the main deco to the modem. Cable 2 is connecting main deco to...

@Alexandre. I'm using the deco in router mode. In the tp link app speed test I'm getting close to 800mbps. What's strange is that when using the following setup, I am able to get just over 200mbps....

So I just set up my 3 deco m5 mesh points. I have 1Gbps fibre and am getting 400+ over wifi from the main deco which connects to the modem. I then connected the second deco upstairs via a Cat 6...