Just wanted to confirm this worked for me also.

1. Delete the network
2. Swap around a couple of the deco's so the one connected to the internet is different
3. Reset all of them again with the pin
4. Setup network again

Everything seems to work perfectly now and my latencies are consistently in the 3-5ms to the router, so definitely more stable than before on wifi only.

Great job and many thanks for finding us a solution.

Update: Tried the reset method and that appears to have worked!

Whilst I am happy to see the powerline symbol I'm still a bit concerned that there could be other firmware or low level problems given what has been said in this thread about how they were flashed during production. Will monitor and let TP-link know as they were going to have me RMA them back for inspection. Or maybe the powerline was working all along in the background and it's just the detection part that wasn't working.

  @ommoti Found "Success to get plc backhaul" but you have to search page by page.

  @David-TP I'm rather suprised that the reset button was needed to get it working, like another user posted maybe it was a missed step during manufacture or some firmware problem.

I'm a little concerned that it might happen again or something else isn't working as should if there were firmware problems or low level soft locking going on.

I've messaged support and let them know as I was going to RMA mine for inspection and try another set.

  @Alexmesh 

Likewise I am concerned whether the PLC backhaul is actually working in the background so I have offered to have another remote session tomorrow morning on two of my laptops - one wireless and one ethernet connected to my office node - so they can verify whether this has indeed fixed the issue.

I'll update when they have some results!

  @Dracon I look forward to reading about the findings. 

Just confirm the update worked for me as well, now confirmed PLC is active.

A couple of points to note:

1.  I rolled the firmware back to v1.0.0 and it is still on this, I'm hesitant to upgrade as setting the router up again is a ballache (I'm lazy) as I have to forward ports to my server as well.

2.  I did a speed test at the furthest deco from the main deco and I'm now getting far higher speeds (maxes my WAN connection) than previously, so it was definitely NOT using PLC previously.

Mine has stopped working over PLC again, since a couple hours. No idea why, haven't changed anything since I went through the hassle of resetting it yesterday. Can't be arsed going through the process of resetting it all again - it works as it is.