ENABL to 1 to enable it.
Connect to the Mission Planner via telemetry. Verify on the bottom right corner of the HUD that you are “flying” in a non auto mode (Manual, Stabilize, FBW are ok).
Unplug one of the telemetry radios. After a few minutes power off your autopilot. (Remember the autopilot will not go into failsafe until 20 seconds of MAVlink inactivity have passed).
Connect your autopilot to the mission planner and pull the logs. Verify on the log that the autopilot went into RTL after 20 sec of MAVlink inactivity.