The issue here is that the DCM communication is paid for by Toyota. They have a contract with ATT. The primary use of the DCM is for communication about the car status that is used by the app. The in-car wifi is essentially an add-on to the DCM. If you want to use a different carrier for wifi, you could get a hotspot device from the carrier of your choice or use the hotspot on your phone.
The only thing you would pay ATT for is to have Internet access via Wifi in the car. This allows people traveling in the car to essentially have a hotspot to access the Internet. Many phone plans will also give you a hotspot, so it is really not required.
There is no ability to access the car data external to the car except through the Toyota app. This does not require paying ATT for the hotspot. You would just have to pay Toyota once the free trial is over.
