A quick update from my side - I have just tried disconnecting and reconnecting a satellite deco unit as suggested above (as it seemed like an idea worth trying). It has unfortunately had no effect. I am on firmware 1.0.3 which is the latest I can seem to update to as of this time.

I originally saw intermittent PLC connectivity upon first connection when I had the units initially, but now this never happens. This might have been on an earlier firmware, which would be consistent with other observations here.

I am stuck as to where to go next personally. My dev intuition tells me this feels like a firmware problem though, as it seems pretty improbable all these deco units have faulty PLC hardware, and I have used old AV600 devices on the same powerline network previously with success.

  @alex12723 Are you in the UK? There was a theme emerging at one point of many people from the UK with this issue and who purchased via amazon but also one was from currys.

Adding a screenshot for future reference, where it shows wifi signals between mesh access points it should be showing PLC+WIFI is what I'm lead to believe going by what I've seen online and read on a TP link article on the subject.

  @Alexmesh 

I have v1.0.  
Firmware: 1.0.3 Build 20230420 Ref 77020

  @Andrew155 Same.

  @Alexmesh same here

UK and purchased around prime day through amazon.

Tried all suggested troubleshooting with no luck. Ran AV1000 before in the same sockets which ran perfect.

Logged with support, hope they can resolve.

I've been told this morning that they have not been able to make any further progress in getting to the bottom of the issue. They have requested me to start an RMA process so I can send them my set for testing. It almost feels like we should be compensated for going to all this trouble to help them troubleshoot/improve their product?

Has anyone had any experience with the Linksys Velop range? Community Fibre (London) provide them as part of their "WiFi in Every Room service". I've compared the Linksys  mesh range and they generally have more antennas and therefore better range than TP-Link

  @Dracon I've been given the same advice.

Had an email back today after contacting support and sending them lots of pictures of the issue and product labels.

They have said they think it is a hardware issue and that they don't think it can be resolved with remote support. They have advised me to exchange them for a new set of units.

They have also said they want my units for testing and to start RMA process

Hardware issue? That wouldn't explain why mine did work, until I updated the firmware. I'm really struggling to believe that all of us have been sold defective units from Amazon, bizarre situation.
 Has anyone successfully rolled back the firmware to any effect?