Apple released iOS 12.1.2 final version for iPhone X, XS Max, XS, XR, and compatible older devices. This minor update fixes some errors for iPhone users.
According to the list of changes, the upgrade solves a problem that prevents some users from activating the e-SIM on their iPhone Xs, iPhone Xs Max or iPhone Xr.
Once activated, the built-in eSIM is combined with the normal SIM card slot to provide dual-SIM functionality for these models for users that wish to have two separate lines on their iPhone (for example, one for work and the other for personal life).
The change list also mentions an error fix for a problem that prevents an unknown number of iPhone 2018 owners in Turkey from connecting to their wireless service provider.
iOS 12.1.2 release notes
iOS 12.1.2 includes bug fixes for your iPhone. This update:
Fixes bugs with eSIM activation for iPhone XR, iPhone XS and iPhone XS Max
Addresses an issue that could affect cellular connectivity in Turkey for iPhone XR, iPhone XS and iPhone XS Max
This update weighs 418.5 MB and can be downloaded via OTA as well as by going to Settings > General > Software Update
So make sure you are connected to a WiFi network and have more than 50% of the battery, so you can install the upgrade without problems.