Cannot connect to FTP server from external host

"Are you trying to connect to the server's external IP from an internal computer? If so, and if you are using a typical home router in a NAT configuration, the problem may actually be the router.

Presumably you've configured the router to forward FTP to the Ubuntu server, and you're trying to test that this works. But when you test the external IP directly from your internal network, your router has to support a ""hairpin"" route. Your router must send traffic from an internal host, out through the external IP, back into the external IP, and finally forward that back to your server's internal IP -- all on the same device. Most consumer routers don't handle this situation properly.

So you need test from a real external host. You have several options:

Use an internet port-testing service like Can You See Me.
Send your test traffic through a proxy.
Have a friend test from their network.
Find a local coffeeshop with WiFi and test from there."