@RahulBilove You can generally login as any operator defined in your app, however depending on the url being used for authentication, you might need to ensure it is not an "externally authenticated" operator for it to work on your device.
here are some things to try:
Delete and re-install the mobile preview app
Try to use the camera of your phone to read the QR code and launch the app in your mobile browser
Debug to see the URL the QR code encodes to verify it's a working URL