Thanks to the ngx_echo module, it is trivially easy to build a clone of the icanhazip.com service with nginx:

server {
	listen 80; 
	listen [::]:80;

	location / { 
		echo $remote_addr;
	}   
}

The Cisco WAP121 runs a Linux based firmware. This is how you get a Rootshell on it:

1. Login to the Web GUI of the WAP121 and enable the SSH management access
2. Login with SSH and enter this command: sh

This probably works with the Cisco WAP321 as well (I only tested with the WAP121).
Also when having still only the one 'cisco' user account configured you can directly get a Rootshell via SSH like this: ssh -l root@<WAP121 IP>