@AshMadden 

You don't need to force Deco node connect over the Internet: if it can, it will. 

Which means, something is not right, because Deco does work over power line adapters. 

You have one powerline adapter connected to Satellite XE75. There must be second powerline adapter to establish Ethernet link. Where is that second adapter connected? 

Try connecting it to Main Deco if it is not. 

  @Alexandre. 

Thanks for the rapid reply.  Actually, I have four AV1000s (all working fine).  One is connected directly to an XE75.

  @AshMadden 

Is one of them connected directly to Main Deco XE75? Please clarify. 

  @Alexandre. 

The AV1000 is connected to the main deco.  i actually tried connecting it to both main and not main.  No difference.  

  @AshMadden 

It is possible that your powerline adapters are not compatible with Deco Ethernet backhaul protocol. It is unlikely, because they are of TP-Link brand, but who knows.

I suggest you run one last decisive test to verify if your powerline adapters compatible with Deco:

Power off (unplug) all powerline adapters. Take two adapters, connect one of them to Main Deco and another - to Satellite Deco. Do not connect anything else to these two adapters, only Deco.

I would also suggest to power off/on Satellite Deco after you connect it through powerline adapter.

If these two adapters can establish connection with each other, but Satellite Deco (even after restart) does not use Ethernet backhaul - they are incompatible.

  @Alexandre. Hi,

Thanks for your suggestions which arrived after I found a resolution (but I'm not sure why it works or if it's a reliable fix).

As mentioned, a different wireless access point had been working just fine in the exact same spot connected via the AV1000.  But the Deco wouldn't.  Even though the AV1000 was showing all connected.

Moving the AV1000 to a different mains socket in the same room did work.  So now I have the Deco connected via the AV1000 and it all appears good.

What I don't understand (and maybe I don't need to if this carries on working):

1. If the AV1000 had a problem with connecting previously, why was it showing a connection (three green lights).  

2. And how did the Apple wireless access point work fine but not the Deco?  

3. Is the Deco somehow more sensitive to a less than perfect connection via the AV1000?

4. If the AV1000 connection wasn't good enough, how could I have known?

Maybe it's one of those things that will never be known.

  @Alexandre. I spoke too soon.  It did work for a bit but now the Deco has gone offline again.  I never tocuhed it between when it was working and when it wasn't; honest!  

The AV1000 is still showing full connection (in and out).  Unless you can think of anything else, it looks liike the combination is a flaky one.  A shame since both products are from TP-Link.  

  @AshMadden 

If you have Main Deco connected to ISP router (not just modem, but router/gateway) you could try switching Deco mesh to Access Point mode. See Switching a Deco to Access Point (AP) Mode

Deco Access Point mode eliminates most of connectivity issues, as long as wired link stays stable.

Other option I could think of is if there is TV cable in your house, and it is going between Main Deco and Satellite Deco. You could use Ethernet over coaxial cable adapters - not cheap, but it is reliable gigabit link, just like Ethernet. If you have TV cable you can reuse, I'll share details.

Other than that, you can still run the test I proposed: have only two powerline adapters plugged, with only Satellite Deco connecting to Main Deco with them. If connection stays stable for a while, you can start adding back powerline adapters and gradually connecting other devices to them. Keep an eye of Satellite Deco backhaul and this may help to narrow what triggers the problem.

  @Alexandre. Hi, thanks for your suggestions.  I've left it running for a few days to see how it went.  My conclusion is that the TP-link ethernet over mains connection isn't as reliable ** in that location ** as I thought.  Perhaps the previous (Apple Extreme) wireless access point was less sensitive to the quality of the wired connection and/or didn't report problems.  

I suspect also the green light on the AV1000 isn't 100% reliable as an indicator of a solid ethernet connection.  It's the furthest point in the house so I guess it's not entirely unexpected there's a problem.  But it kind of works now.

Thanks for all your help.

Ash