1. Log in to server as root user and open default FTP configuration file.

# vi  /etc/pure-ftpd.conf

Search for following line and change it

From
#Bind 127.0.0.1,21

To
Bind 0.0.0.0,121

Note: It means all ip will connect to FTP service on port 121 but make sure that you have opened a port 121 in server firewall and restart the FTP service.