I'm not sure if the streaming definition can be adjusted when using the Fire Stick, but if it can you should try streaming in SD, as this may not only solve the issue, but also save you a LOT of data, as streaming in HD can use a ton of it.
If you can't reduce the streaming definition, or you simply want to stream in HD, try turning off, or pausing, the Video Data Saver. It's designed to save data by throttling your speed when streaming to such that is amenable to streaming in SD, but if the streaming service can't auto adjust the streaming definition and tries to stream in HD it can cause buffering issues. You can turn off, or pause, the Video Data Saver through the Usage Meter (Video Settings tab) or by signing in to the Hughesnet myAccount site and clicking on the "Settings" tab.
With this said, you can check your service speed at testmy.net. When testing your download speed, make sure to choose the manual 25MB download test. To test upload speed, use the manual 4MB upload test.