Before the reset, remove the battery from the door push for about 10 minutes, press the push a couple times to remove the residual voltage and put back the battery to start the reset.
To reset it, press and hold the volume button on the side of the chime for 10 seconds. The chime will beep twice.
Release and hold the volume button again for 10 seconds. The chime will beep once to confirm the reset.
Afterwards you can try to pair them together again.
Take the chime out of the socket, plug it back in, if necessary turn the socket on, the chime will beep.