I'm running 3x Zyxel WSM 20.23.05.0-rc4.
I tried uploading the sysupgrade for 23.05.2 (having used the firmware selector). The upgrade failed and I couldn't ping the AP on any IP address.
I had to go into failsafe mode then ran 'firstboot' and rebooted. I could then upload the backup of the configuration and everything was fine.
Is there a more resilient way to upgrade the firmware? I would rather not have to go through the process of directly connecting to the other two APs in order to upgrade them as they are not as easily accessible (in the loft, outbuildings etc).