I have set my router to forward the port for Mumble to my server machine on the LAN.

External site (and Mumble users trying to connect to my client) report failure:

I can connect to the Mumble server on the LAN without issue. I have restarted the router. Have I set up the mapping incorrectly, or is there an issue with the Mumble server on 16.04?
Edit: I suspect my ISP may be blocking the port, since it's a SIM based network.