Maybe the broadcast script on the Navio/Pi side might help, that I just posted a copy of here:
Otherwise somebody else should answer if you think it is a Ubuntu specific issue. I suppose it could be a firewall on Ubuntu, because the GCS is receiving data from the drone I guess it needs that port opened. On Windows it would have popped-up a user dialog asking to allow the port automatically. Of course on Linux these are the things which are usually manual effort, editing some file of sorts or obscure command to open the port. Maybe Ubuntu has a nice GUI for it, or you can find one in their App store.
There should be something about that on the APM Planner 2 web site.