I don't think it's a server issue (i.e. you can't do anything from the Elgg-side to fix it).
Most likely it's a browser issue. I have no experience with MacOS at all, so I can't test anything myself. What browser are you using? Is it the latest version of the browser? If it's Safari, have you tested if it works with Chrome or Firefox? I also found a posting at https://discussions.apple.com/thread/7683587 where someone also has a problem with html5 videos not playing. In this case it was due to a ClickToFlash extension in Safari. If you have this extension, you might want to try if it works without this extension enabled. Or maybe some other browser extension is causing the problem in your case.
Range support is required for iOS streaming per Apple. For old Elgg, you'll need to write a plugin and have basically two options:
Security issues should be reported to email@example.com!