port forwarding not working

[Stoveclock]

is your router, your modem at the same time, or are you running like this modem-router-raspi ?
Then also open up the ports in the modem.
some modems for some odd reason need to be restarted for the changes to take effect, most of the time it will tell you so, sometimes it just doesn't.

My modem is a modem/router combo and it restarts automatically. I have my raspi connected via Ethernet

[Stoveclock]

Usually when you try to connect from your own LAN to your own external IP the connection won't work, so this could be the case. The external port checker or one of your friends trying to connect from the outside should be the definitive answer.

I suspect an IPv4/IPv6 issue. From the netstat listing, your Cuberite is listening on IPv6. Do you have IPv4 enabled on the raspi (sorry, can't help you with a command here, it likely depends on your Linux distro)?

Should I enable ipv4? I'm running raspbian pixel.

[xoft]

Try if that helps.