I know this is an old thread but kept coming across this one when I was trying to find the answer to my problem. I couldn't find the answers I was looking for anywhere, but found it via my own trial and error. Apologies if this has been answered elsewhere, but thought I should post it somewhere in case someone else finds themselves in the same boat as me.
I'm going try and keep a long story short, and keep it simple. Apart from the parent router connected to the modem, EVERYTHING else in my house is wireless (you don't need to tell me wired is better). I reset everything and did not
I just joined Community Fibre from Virgin Media, both 1Gb plans. CF provided the Linksys SPNMX56 (also known as AX5400) router, and connected it to the modem and I had an extra node. I used to have an Orbi mesh and the VM mesh (on separate occasions) and both worked well. However the AX5400/SPNMX56 Coverage is poor, and they couldn't cover my extended 3 bed semi with a loft conversion.
After some research I bought CF branded AX4200, a non branded AX4200 from eBay sellers and one more CF branded AX5400/SPNMX56. I'm sure I tested every possible combination and location of these routers, as both parent nodes and child nodes, and these are my findings.
The AX/MX4200 is a little slower than the SPNMX56 in close proximity (about at 100mbps), however extends coverage far at much higher speeds. For instance the external garden security camera connected to the parent MX4200 in the front of the house, but couldn't on the SPNMX56.
Having the AX/MX4200 as the parent node (no point using the AX5400), but the AX5400/SPNMX56 as child node upstairs, I had download speeds of 200-220, and upload speeds of 100-130 on the PS5 in the room next to it. Using the MX4200 as the child node I had download speeds of 400-450 and upload speeds of 180-200.
In this set up I had download speeds of 200+ in the loft with both routers so I don't need another one.
Non CF branded only worked as a child node, but worked fine as a child node.
So for my conclusion. If you are going to use a completely wireless mesh, the older AX/MX4200 router is far better than the new SPNMX56 router that CF now provides. Try and grab the older ones if you can.