Ok, here's the quick guide for un-bricking the NWA50AX Pro
You can avoid this by flashing the openwrt firmware the first time you boot up the device, though as another has pointed out, there's no way to guarantee that this is the right slot. You have above a 50% chance of bricking your device and having to connect to the AP via serial cable to recover. Once bricked, I used an ubuntu server to flash the new firmware:
Download a new basic kernel image (initramfs)
Move the file to /tftboot (you may need to modify permissions)
Rename the image to nwa50axpro-openwrt-initramfs.bin
Setting up the tftp server:
sudo ip addr add 192.168.1.66/24 dev eth0
sudo dnsmasq -d --port=0 --enable-tftp --tftp-root=/tftpboot
connecting to the NWA50AX Pro over serial:
Zyxel’s console header pin define: pin1 NA, pin2 White(RX)，pin3 Green(TX), pin4 Black(GND), PS: Don’t connect red color pin to zyxel’s console pin. Pin1 is indicated by the triangle.
Once connected to serial AND connected via Ethernet to a switch on the same lan as your server, open up a terminal and type:
screen /dev/ttyUSB0 115200
Power on the device and interrupt the boot
procedure by pressing Enter.
The bootloader has a reduced command-set available from CLI, but more
commands can be executed by abusing the atns command.
Boot a OpenWrt initramfs image available on a TFTP server at
192.168.1.66.
$ atnf nwa50axpro-openwrt-initramfs.bin
$ atna 192.168.1.88
$ atns "192.168.1.66; tftpboot; setenv fdt_high 0xffffffffffffffff;
bootm"
At this point you should be able to connect to luci by directly connecting to the lan port and accessing the webui at 192.168.1.1. I'd re-flash a new full image for good measure.