Thanks, Ralph and KatolaZ. That was helpful info to learn.

After moving around quite a bit during the experimentation, it's
probably not a hardware or software problem per se unless a different
channel works better or I can find a way to attach a better antenna.

It turns out to be that the environment has changed enough to affect the
signal. Previously I was already at the maximum range with -91 dB at
that spot because of a wall and distance. Since the last few days that
particular spot has become blocked line of sight from the station by
four thick pieces of foliage. Apparently it's enough to drop the
already feeble 5GHz signal down into the "doesn't register" range for
the hardware I have on either end.

So while I don't have it solved yet, it is probably explained by
external factors.

Regards,
Lars