if you have enrolled and upgraded to ios 12 public beta 1. Most probably you'll see there's no "Personal Hotspot" option. If you're facing this problem. Please go through following steps to get it back.
This will remove all network settings and return them to factory defaults.
Launch Settings app → General.
Scroll down and tap on Reset.
Tap on Reset Network Settings.
Tap on Reset Network Settings again to confirm.
That’s it! I hope these solutions have yielded the desired results, fixing the issue. But in case you haven’t found success as yet, restore your device. Make sure to back it up before going through the process as usual.