Try to jump start your 21700 to your 18650.
+ to +, - to - with a paper clip for a few minute, when your 21700 voltage above 2.5 v
put your 21700 in the light and charge it up.
You got a defective battery.
The light will not charge the battery if it is below 2 volt.
Try to jump start it with another battery, you got any 18650?
You can email them for another battery?