Entering your phone's Airplane Mode will automatically turn off your phone's Bluetooth and the watch's Bluetooth. Because of this, the watch will not perform Auto Reconnection when you exit the phone's Airplane Mode. Auto Reconnection also will not be performed if you disable the phone's Bluetooth and then re-enable it.

To re-connect after entering the phone's Airplane Mode or after turning off the phone's Bluetooth

Perform the procedure under "To re-connect manually after a connection is cut".