There is no seperate Iphone mfa setting that i'm aware of, they're all the same basic underlying code.
What you might need to do is reset your MFA seed as i'm not sure the default google authenticator works when you load it onto a new device (some do, but AFAIK the google one requires a new seed).
Go into your forum account page, two factor authentication, make a copy of your "backup" codes (you should have these noted down somewhere safe already), then under "verification code via app" you need to click "manage" , select the "regenerate code for a new app" and confirm then follow the instructions.