Hello there,
I had exactly the same problem..... eventually found that the SIM worked OK for voice and texts but any data connection was blocked. After much head scratching and some help from the very helpful Asda Mobile people - the answers lie in the "Data Connection" settings in the Car MMI system. Note that some PAYG Sims do not include data. The internet connection details in the MMI have to be changed to precisely match the mobile phone Network - the APN Name, the User Name, and the Password all had to be changed,...
I use an Asda Mobile SIM - as there is no monthly charge or limit, so a £10 top up lasts me months. Using Google Earth does clock up the data usage, but I use the head up display for navigation so don't use Google Earth.
My local AUDI dealers were of no use.....
Hope this helps!!