Momentum 313U

I have an ATT aircard. It normally allows it to become a Hot spot. Suddenly after so many years, it will not allow me to use the Hotspot function saying I need a 10gb plan which I do have. The ATT access software shows I have 10gb and my plan on att wireless shows I have it.
The unit software does not seem to have updated.
I have had ATT techs look into the matter and at one time, 1 tech was able to get it to work but the next day it reverted to saying I need a 10gb plan. No tech has been able to fix this. I was sent a new sim card and it was used in the unit but it still doesnt want to allow me to use HOTSPOT.
ATT tells me to take the unit ( i actually have 2 different ones and both are telling me the same thing) to the store and they should be able to fix it but when I ask, what happens if they cant, they said I would have to buy a new unit. This one is about 14 years old and works beautifully otherwise, as does the other aircard but I just cant get them to be hotspots.
I use that function to update my cell phone as I dont have wifi.
This is so frustration. A new air card is $180 when I paid about $50 for each of these long ago.
Anyone have any ideas on how to fix this???