@toomanymatts 

1. Can the Deco E4 actually use Ethernet backhaul via Powerline adapters, or is GPT simply wrong?

Yes, it can. I tested that at my house, but with different brand of powerline adapters. Should definitely work with TP-Link PA7017.

2. If it can, is there some hidden setting that forces the node to ignore the wireless link and use the cable instead?

There are no hidden settings, it should just work. Deco should consider connection to powerline adapter an Ethernet link, and Ethernet link takes priority over WiFi backhaul.

Try different Ethernet cable to connect E4 to powerline adapter, just in case the one you use has issues.

3. If it cannot, is there a Deco model that will use wired backhaul over Powerline reliably in a layout like mine? 

There are Deco powerline capable models, but because it is not clear why your setup is not working, I am reluctant to recommend you spend money on them.

4. Forget Powerline for a moment. Is there a more concrete proof node I should replace the bedroom E4 with?

If you have coaxial (TV) cable terminating in the bedroom, you can use it to establish Ethernet link, but that will require buying adapters.

5. Since I cannot return the Powerline kit the shop policy is that it works, therefore no refund, can I hook up an old router to the Powerline in the bedroom and run a separate Wi Fi network there?

Yes, you can. In fact, try that to see if router works through that powerline link. 

6. Worst case scenario. Do I simply accept awful bedroom Wi Fi and use the Powerline only to hard wire the bedroom TV?

If speed is that low (3 Mbps) and you can't make Deco connect over powerline (which it should), having router broadcast its own WiFi in the bedroom (#5) could be better option.

  @toomanymatts 

One more thing: you have two powerline adapters, one is in the bedroom. Where is second powerline adapter and what is it connected to? This part is not clear from your original post.

Alexandre. wrote

  @toomanymatts 

 

One more thing: you have two powerline adapters, one is in the bedroom. Where is second powerline adapter and what is it connected to? This part is not clear from your original post.

  @Alexandre. 

2. in my bedroom, this is the one that needs playing-with

1. in the 'utilities closet'.  In that closet I have tested it attached directly to the router and to the primary node.  Both worked on the laptop in my bedroom when directly connected, however neither resulted in a connection to the bedroom node.

Used the same ethernet cable when testing on the laptop, so that doesn't seem to be the issue.

  @toomanymatts 

It should work when powerline adapter in the closet is connected to the primary E4 node. It may or may not work if you connect powerline adapter directly to the router.

These are tests I can suggest for you to run, maybe they'll help to narrow down where the issue is.

1. Bring bedroom Deco E4 to utilities closet and connect it directly to free Ethernet port of primary E4 with Ethernet cable.

The purpose of this test is to check is everything OK with hardware. In that simple setup bedroom Deco E4 will use Ethernet backhaul if everything is OK. Also, try with connecting Ethernet cable to one and then to another Ethernet port of bedroom E4: in both cases bedroom E4 should report Ethernet backhaul.

Assuming test #1 is successful, proceed to test #2. If test #1 failed (bedroom Deco uses WiFi backhaul), please let me know.

2. Connect primary E4 to powerline adapter. Take bedroom powerline adapter and plug it to electric outlet on the same circuit breaker, but closest to utility closet. See if powerline adapters established connection and test it with laptop. Bring bedroom E4 to where bedroom powerline adapter now is and connect bedroom E4 with Ethernet cable to that adapter.

See if in this setup bedroom E4 can establish Ethernet backhaul.

If it still does not, configure Deco mesh to run in Access Point mode, see: https://community.tp-link.com/en/home/kb/detail/334

In Access Point mode, Deco mesh disables almost every configuration setting and becomes just pass-through for Ethernet and WiFi traffic. The purpose of this test is to find if there are, even if unlikely, settings in Deco mesh preventing Ethernet backhaul. In case you miss some of features that present when Deco mesh runs in its default Router mode this configuration change is easy to undo after you've done with testing.

With Deco mesh running in Access Point mode, try setup from test #2. To cover all bases, if that setup is not working- connect utilities closet powerline adapter directly to the router and see if that makes bedroom E4 use Ethernet backhaul.

These are all troubleshooting steps and tests I can think of. 

  @Alexandre. thank you very much.  Busy week, so will play around with all this on the weekend and report back.  Assistance much appreciated.