That depends what you mean with subscribing.
If you mean recurring costs: no, there is no recurring cost. Well, there is an exception: If you want access to real-time network data while using the Windows software you have to pay for it.
To use the Windows Software you need a username and password. As far as I understand, these are delivered with the micro RB and are not tied up to your name. Well, as long as you don't purchase it directly via AirNav Systems I guess.
To use RB24.com Website with additional features and without ads you need to sign in / register. After registered you can link your box and see your flight data on RB24.com. All these features are free of charge as long as you share your received data with the Radarbox network. (
https://www.radarbox24.com/subscribe)
The given Information might not be complete.
Ingo