@DJames 

So, after a rather unhelpful online chat with support I discovered that even though ALL the publications say OneMesh devices connects to each other, they don't! That is, you can't have two routers connect even if one is made an access point!

Then I find out the G.hn240 Powerline adapters don't actually extender the WiFi network even though it says they do in the title on amazon... And aren't OneMesh compatible.

I've sent the Archer AXE5400 X75 router back along with the G.nh240 Powerline adapters. And bought 3 TP-Link AX1200 Powerline Adapters with 'OneMesh". Plugged these in and had to wait a good 5 minutes before they both appeared on my network in the MR500 routers admin page under OneMesh. Added them both and now have good reach, a fast ish ping time (40s compared to 90+s) and mobile broadband downloading at 40mb+ and uploading at 50mb+ (Which is really useful as I work in the photography/videography industry.

Lesson learned - TP-Link, please sort out your naming, compatibility list and descriptions, absolute nightmare to work out what goes with what, no common system to the naming that makes sense to an end user.

*Note, the Archer AXE5400 looks like a beast, it isn't! The weakest WiFi range I've ever had! 15 feet away and it drops to two bars. My 10yr old Fitz!box 3490 goes three times as far and delivers file way faster. Also, nothing in my house can use WiFi 6 or 6E just yet and this reserves 200mb of bandwidth for a service that is only just coming to market.