With most services, if you can only connect from localhost you need to look at what IP address the service is configured to listen on. Changing this setting to your public IP address or 0.0.0.0 and restarting the service will usually help.

Can you share some details of what FTP server you installed and configured?

If by chance you are running this FTP service on a local computer rather than on a droplet you will want to look into port-forwarding configuration with your home router/modem and ensure that the port you are using is not blocked by your ISP.