ACCOUNTS RECEIVABLE, NET - Schedule of Accounts Receivable, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Accounts receivable, net
Accounts receivable, gross	$ 23,129	$ 23,305
Less: Allowance for credit losses	(4,811)	(5,297)	$ (5,581)
Total	18,318	18,008
Billed
Accounts receivable, net
Accounts receivable, gross	15,717	17,907
Unbilled
Accounts receivable, net
Accounts receivable, gross	$ 7,412	$ 5,398