So iOS 12.1.2 was released last week. The update seems to have caused cellular data to stop working on some of the iPhones. Thankfully, it doesn’t seem to be an issue that has spread to far. The victims of this issue have complained that they have been unable to make calls, send texts, or access the internet unless they are connected to Wi-Fi. Here are a few possible ways you can address this issue:
(1) Go to Settings in your iPhone to turn off Wi-Fi calling. It seems that something in that setting is conflicting with the update and that could be causing the problem.
(2) Revert back to an older version of iOS ( 12.1, essentially). To do this, you will probably have to connect your device to a computer, navigate through iTunes to device settings and restore from there.
(3) Contact Apple Support to see if they have any solutions that can help you out.
Hopefully some of these issues will help you out if you are experiencing this issue. And hopefully the next version of iOS will fix these issues which will hopefully be in the near future.