If you plan to order a new iPhone XS, XS Max or iPhone XR, do not install the beta iOS 12.1 now.
Apple has not yet released for these smartphones a beta version of the operating system. This means that you will not be able to recover from the backup.
Data backups are incompatible with older versions of iOS. Therefore, you will not be able to return the saved information.
Curiously, the final build of iOS 12 is also not available for the iPhone XR. But this is certain to be corrected for the release of the smartphone on October 19th.