I installed the openwrt-ramips-mt7621-zyxel_nwa50ax-squashfs-factory.bin firmware which appears to have bricked the device. On boot there's a solid green light, no access to the ethernet port, and no default wifi to connect to.
The installation guide says "If OpenWrt is installed to slot B, it will fail installation, and reboot to the stock ZyXEL firmware, in this case, simply install a stock firmware from ZyXEL's site, and then attempt to reinstall OpenWrt." which might be what's happened, but there appears to be no rebooting to the stock ZyXEL firmware.
I have tried holding the reset switch down, but didn't expect that to work as that only factory resets configuration changes. I would be happy with either stock firmware or openwrt at this point. Any suggestions with what I can try next. There is a console port on the back of the device, but I've not done serial connectivity on a device like this before.