OK - ftp access from the internet is a little more challenging than from across the LAN, but taken one step at a time, it's not too difficult.
As before, the first step is going to be finding the server. This time, it's not going to be as easy as setting a static ip address - at least not for most of us, simply because we don't have the luxury of a static address.
The easiest way to deal with this is through the use of a dynamic DNS server, such as DynDNS - this allows us to use a URL (a Universal Resource Locater - or in english - a name) to point to our changing ip address. There is a possibility that this has already been setup for you by that student - so you need to let me know how you were accessing the old ftp server - if there was a previous DynDNS account it may still be working, depending on how it was being updated - it may have been done from the router (which is my preference), but it can also be done on the DNS-323, so that is some something we might need to look at.
The second step is to "forward" port 21 to your ftp server - a little explanation is in order here.
Your router/firewall, by default, allows all outgoing connections, and keeps track of them, so when it sees data coming in from the internet, it can match the incoming data to an outgoing request and knows which computer on the network to send the data to. If there was no outgoing connection, when it sees incoming data traffic, it has no idea where to send it, and so it discards it.
For your ftp server to be accessible from the internet, you have to tell the router to forward all incoming traffic on port 21 to the ip address you set on the DNS-323 (this is where that link that gunrunnerjohn provided will come in handy). There may be an old port forwarding entry from the previous ftp server setup - just go ahead & delete it and then create the new one, you'll know if its for ftp because it will use port 21, which is the default ftp port.
Now - once you have forwarded the port, you need to test access from the internet - it should work at this point, but you need to check it from OUTSIDE of the network to be really certain.
So - what you're going to do for me now is tell me, if you know, what the URL you were using to access the old ftp server was - and we will check that to see if it's still working, and you will set up the port forwarding on the router.