If you try to sign in on a new phone, a confirmation code should be sent to your previous one, assuming you still have access to the app on the device.
Or you can just confirm the access from your old phone:
You will see this widget on your phone > 1ST OPUS app:
If you can't access your previous phone for whatever reason, please try to recover your account by following the steps in this helpguide.
Please note that the recovery process is needed when reinstalling the app or trying to access your account from a new device. You will not need to go through it if the app has been updated by us :)