Enable Developer Options on Samsung Galaxy Tab S 8.4
This is a bit of a desperate idea, but try rotating your device between portrait and landscape. It doesn't seem likely, but it's possible somebody at Samsung may have accidentally left out the Developer Options item from the layout or changed something that caused it to remain invisible.

If that doesn't work, try installing an app from the Play Store that acts as a shortcut to developer settings (like this one). It's not the most elegant solution, but it should be enough to get you there.