https://www.theverge.com/circuitbreaker/2018/10/31/18047890/iphone-dual-sim-support-esim-tmobile-att-verizon-not-ready-ios-12

Apple released iOS 12.1 yesterday, adding support for a second SIM card to the iPhone XS and XR through the use of a digital eSIM. Apple may have technically added support for eSIM cards, but none of the major US carriers are ready for it. According to a report from MacRumors, AT&T, Verizon, and T-Mobile are all delaying their adoption of eSIM until later this year. (Sprint wasn’t included in Apple’s original announcement, but it told PCMag that it will support eSIM at some point.)

So if you’re a US cellular customer, you can’t actually use a second SIM with your phone unless you’re using one of the international carriers that do support eSIM.