Browse to the Meeting Info page in the Skype Meeting Broadcast portal.
Scroll to the bottom of the page to the Media distribution heading.
You should see the text Software-Defined Network is enabled.
If you do not see this test, it's possible Hive Streaming was not properly enabled on the Skype for Business / Office 365 tenant. Check with your Skype or Office 365 Administrator. It's possible the tasks to enable HIVE for Skype Meeting Broadcast were not completed.
It may also be possible the event in question was scheduled before HIVE was enabled.