One possibility is that your APN settings haven't been set up(for iOS).
If you have an iPhone or iPad, launch the Settings app and follow these updated steps:
Tap on Cellular (or Mobile Data).
Tap on Cellular Data Options (or Mobile Data Options).
Tap on Access Point Name (APN) under Cellular Data Network.
Then, ensure that it says "wholesale" next to APN under Personal Hotspot. If it doesn’t, you can edit it accordingly.
Save the changes, and your hotspot should work. Let me know if this works for you!