Hi Ryan, I have a 2017 A3 sline with Audi connect. It came from new with a 3 month subscription which has been renewed by Audi uk for free for 12 months. If it does not appear in your MMI menu then I don't think you can add it later. It does not activate Sat Nav but allows you to add Google earth overlay to the Nav which looks great. It also gives online content such as news, weather, twitter etc.
I tried to add the Nav to my sons A1 Sline and came across the same problem you are finding. You can get the maps on the SD card easily enough but there is no way around paying Audi to activate the Nav. I understand they plug it in and connect to Germany who then activate it.
I bought him a tomtom.
Sorry