Well, as you know I've posted that my Xs Max is performing well in AT&T fringe areas, and faster than my 2017 Qualcomm iPhone X when in weak signal areas and strong ones. And, it switches between LTE, 4G and 3G seamlessly so I have not lost a call yet.
So, today I was in Breckenridge Colorado, and drove the back way through the mountains from Colorado Springs with fairly weak signal in most places on the way there and back.
I had my 2017 Qualcomm iPhone X connected to CarPlay, and also my iPhone Xs Max on the dash vent as a spare mapping device, while using the Pioneer W8400 built-in Map app (since maps are stored on the device and it doesn't need cell to load maps).
Many times I had 1 bar LTE on the Xs Max when the X had no signal (or 2-4 bars 4G when LTE signal would get lost or weak), and I could use the Xs for Siri to send dictated texts or load street maps the entire drive, meanwhile my 2017 iPhone X could not connect to Siri or load any map data in those same places.
So far, if I had not read the messages on this forum, I'd have no idea that the iPhone Xs Max has cellular connection issues. My Xs Max would give me 54.2 mbps down/16.3 mbps down in the center of Breckenridge, while my iPhone X was at 33.1/18.7 mbps.