You can change and turn off each connection type by pressing the home button, swiping up (for iPhone X and newer you will swipe down from the top right of the screen), and pressing the green antenna button for a cellular connection, and the blue wave button for a WiFi connection. Which one you should use and which one you should turn off depends on your event and location.
If you receive a yellow "NETWORK ERROR" screen when attempting to scan a ticket, this means your cellular or WiFi connection is poor and the device is struggling to communicate with the server to verify the ticket. Be patient, or attempt to toggle between a cellular network or WiFi. Once you're connected again, double check that the ticket you attempting to scan has been redeemed by the history arrow.