You will only need a data connection (Wi-Fi or network) to download the app and to purchase tickets or subscriptions. No data connection is required when travelling with the app. Once you have purchased a ticket or subscription, you can activate it offline via the app and use it, e.g. when you travel by bus or train. No data will be used.
If you have a subscription, your phone must be connected to the internet at least once every 24 hours in order to renew the code and colour. These act as identification tools for the driver. This happens automatically, e.g. when your phone is connected to your home Wi-Fi network. You do not have to open the app itself to do this.