The Zyxel NWA1123-AC-PRO is a seriously nice AP (with client isolation! . It does have a serial port, but I could *NOT* find any doc on the configuration for it. The openwrt reference below lists a pinout for a different model as follows:
1-GND 2-RX, 3-TX 4-VCC+3.3v
I started with the assumption that these two units were similar enough that the pinouts were likely the same. I metered the pins on mine and easily verified which pins were 1 & 4, which then implied pins 2 & 3. Given vcc at 3.3v, I concluded that this serial port is probably TTL-level serial *NOT* *RS232*!! (Please NOTE: RS232 voltage levels will probably cook your serial port!) I used the adafruit cable (also reference below) and was easily able to talk to it.
Looking at the back of the device, with the network cable exiting towards the bottom, the pins are numbered 1→4 from LEFT to RIGHT. Said another way, pin ONE is closest the EDGE of the device while pin FOUR is closest the network ports. The adafruit cable is connected to pins 1, 2, 3 with colors Black, Green, White respectively. Pin 4 and the RED wire are left unconnected.
The default settings are n/8/1 at 115.2.