These steps fixed the error "Unable to connect - Data connection not available". I had already validated hotspot was enabled on the account, that the phone did have LTE data (and that data was working properly), and rebooted.
1. Open Settings and go to Apps
2. Click the triple dot and Show system apps
3. Search for Tethering and do the following steps to both "Tethering Provision", and "TetheringAutomation"
a. Force Stop the app and go into Storage and Clear data
4. Mobile Hotspot should now connect