Which phones are eligible for hotspot plans?

Hi,

I have a general question for my next phone purchase.  I tend to get premium plans with hotspot capability.  Assuiming I continue to do so, do I need to buy a phone offered through ATT as hotspot eligible to be able to hotspot (choice one and most expensive) something to it, can I buy any phone through my ATT account and it should be eligible as long as my plan is hot spot eligible (mid choice), or can I buy any phone from a third party, so long as it is unlocked and voice and text and data work normally and it should be eligible for hotspot if the plan allows?

I've had an experience at the stores that seems to suggest I have to buy  a flagship phone to get hotspot, but the two flagship phones I have now have a host of issues.....

Thank you