Configure WR21 to communicate with serial0-RS232 on port 9999 through mobile at public static IPv4 adress?

Need to communicate with some RS232 equipement behind the WR21 throug mobile interface to Serial0 through port 9999. I have a static public IPv4 adress on the sim card. How do i configure the WR21?

Change the default port number of asy 0 in

Configuration > network > network services

or CLI

cmd 0 asy_listen 9999