It got worse.
I reset.
The only light on was the power light which is solid on green.
I stood with my phone, the extender and the router all within a 1 metre radius.
During the set up process I got as far as putting in the passord for my router for both 2.4 and 5 and click next.
After approx 22% of "Applying your settings" the app just displayed "No Main Network Connection"
My options were "Start Over" or "Retry"
Whichever one I selected I ended up back at the same position with this page on the app.
I already had one of these devices upstairs. I turned it off so as not to cause a conflict.
Then it got better.
I started to try differing configurations and the one that did it was to skip setting up the 5g.
The device now works on just 2.4, even though the light it on for 5 as well.
Not perfect but it will have to do.