OMG I just found a solution on my iPhone 11 on Australia's Telstra network. For anyone else with the issue:
After spending days on support to both Telstra and Apple, i tried the below:
-Removing/Replacing SIM card.
-Trying another Telstra SIM/Service of a friend in the same phone.
-A full recovery mode wipe of the phone to factory defaults.
-Network setting resets.
-APN UN and PW entry and subsequent reset to defaults APN only of telstra.internet which is the same on my friends phone which works on iOS 14.0.1.
-Trying the SIM card in my old iPhone 6 (where the hotspot works).
None of these worked.
Then I looked at the APN setting of my iPhone 6, just for the sake and I spotted it was set to use telstra.wap.
If I set it to telstra.internet the hotspot worked still.
Back on my iPhone 11, if it set its APN to telstra.wap it also works!!!!! YAYAYAYAY!!!!
Note that if it resets back to telstra.internet again it doesn't work, so its something I may need to keep an eye on going forward....