Did you setup Instagram 2FA in your DUO app, and then changed phones?
When you changed your phone, how did you move data from the old phone to the new phone?
For example, if you have an iPhone, did you do a backup-> restore or transfer your data between your phones, or did you just set up the new phone as a fresh device?
In your old phone, in the settings in the DUO app, did you have "Backup third-party accounts" enabled?
Documentation on how that works is here: https://guide.duo.com/duo-restore
The options you might have at this point are:
#1 when you enabled 2FA for the instagram account, you were also given backup codes to use (onetime use each) in case you lost your authenticator client. Assuming you saved them, use them to log in and change the multifactor authentication settings and re-enable on your new phone.
#2 Boot up your old phone, if you haven't returned/destroyed/reset to factory settings, use it to log into Instagram, and re-go through the MFA process to enable this on your new phone.
#3 Contact Instagram support for help?
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