Go to Settings - Mobile and look at the middle of the screen where you see Network Mode and Network. Has your network name disappeared from the Network field? If so, touch that field and on the next page, hit Set.
Although my network selection is set to automatic, for some unexplained reason it disappears sometimes. I might still see “4G” displayed in the notifications area, but calls and texts don’t go through.
No idea why this happens, unfortunately.