I rented a BMW from Sixt in the USA earlier this year. I wanted to use the ConnectedDrive features, but it was blocked by BMW because the vehicle VIN was (correctly) registered as a Fleet Vehicle (i.e. a rental car) and thus none of those features were allowed with that car.

I have rented BMWs in the Netherlands and don't recall being able to use these features either.

Thus you seem to have encountered a situation which BMW and Sixt know about and have procedures in place to prevent, but their Italian subsidiary seems to have missed it with a certain batch of fleet vehicles, or just this specific one. I'd report it Sixt and move on.