Aura must be connected to WiFi at all times in order to display photos. This is important so that Aura can always show the latest photos on your frame as you take them, and also so that it can download software updates to the frame.
If your frame loses its WiFi connection, it will continue to display photos for 24 hours without WiFi, then it will display a message alerting you that the frame has lost connection. This can happen if your WiFi network information has changed or if you set up the frame in one place and then move to a new location. It's easy to reconnect the frame to WiFi using the Aura App.