The new iPhone XS and XS Max may have inherited the iPhone 4's poor cellular connection strength. Several users are complaining of poor or dropped cellular and WiFi connections. While these claims are still just conjecture, they are rapidly growing in number.
https://www.notebookcheck.net/Antennagate-2-0-The-iPhone-XS-and-XS-Max-users-are-complaining-of-poor-cellular-and-WiFi-connections.334706.0.html
How can it be carrier-specific when WiFi connection is also poor and it has nothing to do with the cell provider?
Seems to be it's indeed a hardware issue with the phone.