In frustration, I updated the
cron script to reboot the RPi Zero if it is unable to establish a connection to the main router. It is a little heavy-handed, but I was getting really frustrated that the baby monitor was not connected when we needed it. Perhaps I will find a better method of re-establishing the WiFi connection without rebooting the RPi Zero.
Our network has stabilized since I added this hack, and the RPi Zero has not rebooted yet. Now that things has settled, I am sure I will forget about this hack. Finding a better solution would require connecting the RPi Zero to a keyboard and mouse so I could test this properly, I just need to find some time. I do have other RPi Zeroes, so I do not have to experiment on the baby monitor itself.