That’s really frustrating, especially for such an expensive device. Since UPS has confirmed delivery on August 11, I’d stop dealing with regular support and ask specifically for a supervisor/escalation team.
Give them the service order and tracking number and clearly state that Microsoft has been unable to locate your device despite confirmed delivery. Ask for a written resolution—either locate and return the device, provide a replacement, or issue a refund.
If they still don't respond, keep all the emails, tracking proof and case numbers. That documentation will be important if you decide to file a formal consumer complaint or pursue small claims.
Try this X-handle:
@MicrosoftHelps