Hi @minjun
I’m sorry to hear that this happened to you. Did you previously add and verify a mobile number for your Instagram account? If so, you might be able to gain access by logging in via a desktop or mobile browser, and when prompted for a six-digit passcode, choose to send the code by text message instead.
Otherwise, unfortunately without the recovery codes it’s difficult to recover, and Duo is unable to help in this situation. Your best bet is to report the account as lost to Instagram by selecting “Need more help logging in?” on the login screen and following the instructions.
It may take awhile to get a response from the team there, but they can verify your identity and disable 2FA for your account to allow you back in.