Well, I pulled the whole set-up apart and put the other router in. It connected, but the SNR was very low and kept crashing. However, as is typical with this house, there are a multitude of reasons it might have been doing it to thwart and generally irritate me.
The master phone socket is right near the power entry point, where all of the breakers and meter are, plus loads of really old noisy circuitry from eons ago.
The router is 30cm away from all of this (nowhere else to put it), plus the power brick it is attached to is connected to a power point that is down about 5cm away from all of that stuff. So it wouldn't surprise me if the combination of multiple power bricks, noise from the power system, and having the cupboard all open is what broke everything.
Also when I went down to take it apart, I noticed that someone had the microwave on, which is not far away and very effective at causing interference.
At any rate even when I manually tweaked the SNR it kept connecting at the same speed! It wouldn't surprise me if the router was broken and corroded somewhere, as I was forced to store it in the damp garage for at least 3 months over the winter.
So I'm back to the standard router for now, at least stable. Generally annoyed, my other router is obviously duff - or my procedure to try and test it exposed the system to loads of noise and broke everything