How do I allow my vsftpd (Linux FTP Server) to be accessed by WAN?

It's for a project I'm doing at work to overhaul our network infrastructure. I built a server and have been deploying new features on it for a few weeks now.

Im trying to get an easy solution for our Sales department's reliance on cloud storage, and in the process make our in-house operations more robust against internet outages.

If anyone knows about this kinda stuff, I'd appreciate some advice.

Electronic surveillance hardly inspired reverence. Perhaps fear and obedience, but not reverence.