How do I fix iPhone SE celluar network after 12.1.2 upgrade?
You need to update your cellular settings.

How to do so from this Apple Support page:


  
  Make sure your device is connected to a Wi-Fi or cellular network.
  Tap Settings > General > About. If an update is available, you'll see an option to update your carrier settings.
  


If there is no option or it doesn't work then:


  If you see either of these alerts, take your iPhone or iPad to an
  Apple Authorized Service Provider or make a Genius Bar reservation at
  an Apple Store. You can also contact Apple Support.


According to this Apple Support page.