According to the commit message, there should be the command zyxel-bootconfig in OpenWrt. You might want to try:
zyxel-bootconfig /dev/mtd10 get-status
to get the current status. If this works and reports a valid image in the other slot, you should be able to change the active image with
zyxel-bootconfig /dev/mtd10 set-active-image 1
NB: I'm not familiar with this device, I'm just interpreting the commit message.