Start by checking that you have correctly set-up your iPhone hotspot - ensuring that both WiFi and Bluetooth are enabled:
How to set up a Personal Hotspot on your iPhone or iPad - Apple Support
This support page generally provides the most appropriate troubleshooting for hotspot connection issues:
If Personal Hotspot is not working on your iPhone or iPad (Wi-Fi + Cellular) - Apple Support
If you continue to experience difficulties, you might seek assistance directly from from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively and perhaps easier, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad or iPhone, the App can be downloaded from the App Store:
https://apps.apple.com/gb/app/apple-support/id1130498044
You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to provide assistance:
Genius Bar Reservation and Apple Support Options - Apple