Plex add-on: how to expose additional ports?

I’m trying to get the plex add-on to see my HDHomeRun for live tv/dvr. The auto discovery doesn’t work, neither does trying to add the IP manually. I read on a plex forum that I need to expose port 65001 udp. In the configuration page I see that I can alter the existing port entries but can’t add additional ones. What’s the best way to expose additional ports? Thanks!