Hi Kelvin8603, please ensure that your mobile setting are enabled. iOS users must ensure that your phone supports version 13.1 and above.
For iOS users, please follow the below steps to set up before trying to turn on mobile hotspot.
Step 1: Make sure the iPhone is updated to iOS version 11.0 and above
Step 2: Go to Settings> Mobile Data> Mobile Data Options > Mobile Data Network> Under Pesonal Hotspot, update APN to shppd (Prepaid) / shwap (Postpaid).