I think you may be conflating two issues. First, the MyAudi app NEVER connects directly to your car. It doesn't matter what phone Service Provider you are using. Your app uses your phones wifi service or data to connect to your MyAudi account in the Audi Cloud. Your car, completely independent of that, uses its wifi/data capability to also connect to your account in the Audi Cloud. The phone and car then link with each other in your Audi Cloud account. For this it does not matter where you or your car are located. If you open the MyAudi app on your phone and click the little update circle in the upper right, your phone should communicate via the Cloud and the Cloud should send you the data it has gotten from your car via the car's wifi connection. As Chasing Cars notes above, you should be able to do this from the other side of the world, if you have wifi connections for both. If you can never get the MyAudi app to update your info, or for you to execute a command, two possible causes come to mind: 1) your car and phone are not properly sync'd with your Cloud Account; or, 2) when you request an update, the Cloud sends a request to your car to wake it up and send that data--possibly your car is having trouble waking up.
You may be conflating this issue with another--connecting to your car's wifi hotspot. New cars generally come with a free trial of all the Audi Connect subscription features. Among these is giving you access to the Audi Wifi hotspot available from the car. For this you do need to be near the car with the car turned on. Your phone can then connect to the car directly and you can access wireless Carplay, Android Auto, and other features via the MMI.