You do not need an active Internet connection to play DoubleDown Classic Slots. Once the app is downloaded and installed, you are free to continue playing on or offline.
If you run out of credits while playing offline, you must either reconnect to make a purchase or wait until your next Bonus Credits are available to collect. The Bonus Credits timer is not affected by your connection status.
It is always necessary to connect to the Internet for app updates. If you are playing offline continuously, you may be missing out on new content.