The iPhone has had a bumpy couple of months. iOS 12.1.2, released last month, broke cellular connectivity for some users. The problem got so bad that Sprint issued a warning to its customers to wait on updating. There doesn't seem to be any relief in sight; not only does iOS 12.1.3 not bring a fix for the cellular issues, but the latest update has actually broken WiFi for several users in addition to cellular connectivity.
https://www.notebookcheck.net/iOS-12-1-3-may-break-WiFi-in-addition-to-cellular-connectivity.401381.0.html