If you opt to use the Duo Mobile app you may run into an issue where your smartphone isn't supported. You can see the supported platforms on the Duo Guide.

If Duo no longer provides support for your device, the Duo Mobile app will still run if it is already installed, but there will be no more updates to the app and it will not be available for download from the app stores for devices running these versions.

If your smartphone is not supported, you can still enroll your device as a mobile phone. This will allow you to authenticate when accessing the 2FA Account Management to request bypass codes.