Does your machine have a cellular modem that gets prioritized only when there's no route to some well-known service via the normal network adapter? And you disabled it (but forgot to mention doing so in this story) around the same time as swapping routers?
Nope! Only WiFi and Ethernet. I had been using my phone hotspot, but that gives me a sane IP in the local reserved block, not one from the public block.
I also was using our neighboring business's guest WiFi, but again that should have given me a sane local IP.
Every diagnostic I could think of told me I was directly connected to the WAN with no intervening networks. Then again I'm not the best at debugging networks so I could be mistaken on this point. I am 100% certain that the IP address my computer was given was not a legal local network address.
That's about the only thing that makes any sense. Maybe it somehow faulted to route all traffic straight to the ONT, which could only give one address which I happened to get. And then the fiber got cut the very next day around the same time as we got the new router.
A lot of coincidences and extraordinary edge cases, but it's plausible I guess?
