Unless you have some sort of restrictions enabled, or your device configuration relies on an MDM/Profile configuration, the Personal Hotspot feature (whether it’s available and can be use it or not) it’s controlled by the Carrier and it’s settings.
So contact your carrier again and ask to speak with someone from higher tier of their support who can look into the issue you have.
Check on your iPhone for an update (if any) to the Carrier Settings. Then you can check if your carrier is one of Apple’s partner carrier and what feature do they support and control.
Manually update your carrier settings on your iPhone or iPad
https://support.apple.com/en-us/HT201270
Wireless carrier support and features for iPhone in the United States and Canada
https://support.apple.com/en-us/HT204039