Searching for that error revealed that I had to install from scratch instead of upgrading, due to a change in how networking is configured. So I decided to postpone updating until I had plenty of time.
Today I had plenty of time. And spent it.
Instead of thinking, I uploaded the second package I downloaded when I read the announcement and just overrode all the warnings. Bad move - the access point didn't come back online after that.
I had chosen the wrong package - I should have taken the "KERNEL (SQUASHFS)", but instead I went for the "KERNEL (INITRAMFS)". I'm not sure how I made that mistake, but, alas, I did.
After trying to trigger failsafe mode and performing a factory reset when that didn't work, without any luck, I took the access point apart and located the place to solder on pins for serial port access.
Luckily I found a nice slide deck showing exactly how to figure out which pin is which with a multi-meter, and I dug out an old serial-usb dongle, some wires and my basic soldering iron.
However, soldering wires onto pcb's is not something I have attempted before (I've used the soldering iron to make a cable for my Amiga 500 to make the keyboard "external" in the 1990's, and then I've cobbled together a cable for controlling my amplifier, and that's my complete soldering experience). So before going further down that route, I looked into one more debricking strategy, using the nmrpflash tool, which conveniently is readily available in Debian.
I didn't have high hopes, because I saw no evidence of the access point jumping into the network with tcpdump, but I thought I might as well try it.
In the first attempt I saw something transferred!
It ended with an error, so I tried a couple of more times, which also ended with errors, but something worked, and the access point started blinking, and then came up with OpenWrt 21.02.0 on 192.168.1.1!
After configuring it and testing that it worked, I put it back together
and hung it where it belongs. I even got
opkg update working, so I
could install the
crelay-package that I use to automatically control
the power to my
It could have taken 5 minutes, if I had taken the warnings seriously and double checked the file I was uploading. Instead I spent most of the afternoon recovering from my mistake. But it's working as it should now, so all's well that ends well, I guess.