System updates are checked in detail before being released to all users, at least that's official statement of producers. It's hard for us to believe that systems are updated in such a way that after installing the latest versions of the software phone may have problems with the easiest tasks. That's the problem which occurs in some iPhones after installing the recent iOS' update to version 12.1.2.
Many users inform Apple that after installing the latest system update their phones can't use the LTE connection. Some of them can't even make phone calls and send text messages. Interestingly, iOS' in 12.1.2 version was announced as update which had to solve all the problems of the previous version of system that didn't allow for using of the Internet through data transmission. As you can see, update that was supposed to fix this problem further aggravated situation. If you haven't updated your system on iPhone yet, we recommend you to wait for the next update.