Hi @Francoly, let me see if I can help out here!
In order to set your phone to update the system automatically while using mobile data, find and tap Settings > About phone > Software update > Tap the menu icon (three vertical dots) in the top right corner > Settings > Auto download system updates > Set to "ON".
Do note that system updates can consume a great deal of mobile data, it is therefore advised to download system updates using a WiFi-connection.
I hope that helped!
