Just a quick one, you CAN use the phones data connection even when connected to WiFi on many phones.....just need to enable it in settings. You don't need a separate data plan for the car, either with Cubic Telecom or via a third party Sim card .
Most phones by default assume a WiFi connection is the connection to use for internet access......but in android and I believe iPhones too, you can switch on a setting that allows the phone to use either or both....android calls it Smart WiFi switcher. It's in the WiFi Preferences menu .
This basically allows me to connect to the car with BT for the handsfree, WiFi to the car for Amazon Music and internet radio, whilst using the phones LTE connection to access Amazon and the Internet radio provider.
The myAudi app still crashes out occasionally for me though and needs to be manually restarted to get Amazon or Internet radio working again.....so not 100% yet .