@AndrewF 

AndrewF wrote

Thanks, I had already followed the advice in this doc before posting. I think i might have to send them back and drill hole for a cable sadly

If you have read that document you should have seen "Powerline Backhaul doesn't need to be turned on by any hardware or software switch, and it takes effect automatically" in it.

Powerline link takes priority over WiFi, which means: if P9s could connect over powerline they would.

Powerline technology is very sensitive to electrical wiring and what else is plugged. I was in the same situation: tried powerline adapters and they wouldn't work reliably at my house. Ended drilling holes in the walls and ceilings to bring Ethernet cable where I needed it to be. Spent time doing that, but have no regrets.

Just make sure you tested Ethernet cable before you pull it through the walls, especially if it is the long one.

By the way, you could still use P9s with Ethernet cables, Ethernet link takes priority over powerline.

Before you sent P9s back (if you decided to), TP-Link also recommends make P9 Main Deco. Chances are slim it'll change anything, but why not to try that? Changing the Main Unit for your Mesh Network