Reset Bluetooth for Plantronics BackBeat FIT 2100 Series
Try holding down both the Power on button and the Volume button at once. You should see the light flashing red/white (or blue?) Now your device should be available to Pair on the Bluetooth device list.
This link might help:
https://www.techwalla.com/articles/how-to-reset-a-plantronics-bluetooth
Worked for me!