Welp, my #router's #LAN ports seem to have packed in. They're all reporting "A-OK over here, nothing to see", but try connecting anything to them and suddenly they're all "None shall pass!"
I've checked that it's only, and all, the LAN ports.
Luckily I already had a new router which I'd been commissioning leisurely like with a view to moving to #VDSL. This adds a little urgency to that effort.