I don't think this can work the way you want, if my understanding is correct.
I assume your intention is to continue to use the Audi Connect services that you had during your 3-month trial. But this is not just about a data connection, you also require a subscription to Audi Connect services as such, and I think a data connection comes as part of the package. This also means you can't use a 3rd-party data SIM, you will need to use the mobile network tied to Audi.
I may not have got this completely right, and you need to check with your dealer. When you've done that, it would be helpful to post what you've found out .