Follow these steps to make the Developer Options appear again:
1. Select FireStick Settings.
2. Click My Fire TV.
3. Click About (only if you DO NOT see Developer Options on this screen).
4. Click on your Fire TV Stick quickly 7 times.
5. You will see a notification at the bottom saying No need, you are already a developer. That’s it, you have enabled Developer Options.
6. Press the back button once or go back to Settings > My Fire TV and you will now see Developer Options.
Now you can go into Developer Options and enable Apps from Unknown Sources to sideload APKs on your FireStick device.