Since the user have removed the Hexnode MDM agent from the device, you will have to re-enroll them to add it back to the Hexnode. You can re-enroll the device by retaining the configurations set on the device:
Navigate to Admin-> Enrollment-> Re-enrollment Options.
Select the option Retain configurations and change owner.
The device can now be enrolled again. Please check out our help doc on enrollment settings for more info.