Several sessions with Telstra service techs later, and an overnight “watch” was able to convince them that sure enough, my modem had been in yoyo mode. Their remote-end tests, and the ones also done on behalf of NBN infrastructure all suggested that the network side of the wall socket was “fine”, and that the problem was therefore at my end. Suggested I try plugging/unplugging everything and hit all the resets.
Now factory-resetting my router is not something I’ll undertake lightly, because it has quite a bit of configuration and setup that doesn’t seem to have any way to save and restore. Resetting the NBN modem seemed do-able though, and I’ve done that, along with power-cycling it and holding a prod on the “Reset” button for a goodly while. Took it a while to get going again, but it’s up. The router is happy about it. Took quite a while to grab an address from DHCP this time, which is perhaps a good sign? That first one only gave a one-hour lease, but I’ve checked after that hour and not only has it been renewed, I have a one-day lease to go on with.
So, unless there’s some physical badness that I haven’t uncovered yet, I think that I might be up and going again.
Yay! Not a moment too soon. Packing is underway.