In-game: Open Settings from the menu → Go to the 'Other' tab → Under 'Customer Support' scroll to 'Redemption Code' → Enter the code in the window → Select 'Use' then claim from Mail. You can also redeem online on the official page (see the “How to redeem” section above). Codes are case-sensitive; one code once per account. On iOS the in-game redeem button is missing — you must use the web.
Why isn't my Mongil: Star Dive code working?
Usually because the code has expired, you've already redeemed it, or there's a typo when entering it.
How often are Mongil: Star Dive codes updated on KodeGG?
Automatically every hour from official and trusted sources. Expired codes aren't deleted — they're archived into a database.