tethering/hotspot temporary network error

kyocera e6920, at&t version 6.103AT, android 10

provider - red pocket mobile

unlocked using the online tool on the at&t website.

i bought this phone new last year.  the tethering/hotspot has never worked.  it always gives a temporary network error and to try again later. other network, data, browser functionality works fine.

i can take the sim card from this phone and put it in my old samsung phones (s5, s4) and they all work just fine for both hotspot and tethering.  therefore showing that my plan and account have tethering/hotspot active and data available.

therefore the problem is with the kycocera at&t phone.

so AT&t representative...  does at&t prevent hotspot/tethering from working if we do not use at& plan? even if the phone is unlocked, and we did not buy it from you in the first place?  if so then how do we fix this problem so that I can may use my phone to the full extent of its capabilities? 

if that is not the case then what further log files and information do you need to figure out the problem?

thank you