iPhone 12ProMax

If one Xfinity store does not have any Iphone 12, does that mean all stores in my surrounding area also don't have the iPhone 12? Mine, in Hendersonville, TN, does not have and does not know when they will have any of them. I need to upgrade. I purchased my wife's directly from Apple. She is using it now. My dad received his today from AT&T. Why can I not upgrade my phone with Xfinity Mobile? Longer I wait, quicker my 11Pro Depreciates.