Can you post a picture of the inside of the door?
The LED does not show a pairing mode. It is always ready to pair when awake.
From the manual.
The LED flashes green for 5 seconds to indicate activity after 2 revolutions.
The unit never does a red flash as far as I know. Where are you getting the information that it should give a red flash?
If it was previously paired then it should still be paired. Changing the battery does not affect the pairing.