The antenna may have become dislodged. (1) While sitting down, turn your phone screen facing down in a horizontal position across your upper leg. (2) Firmly rock it left-to-right across your upper leg for about 5-10 secs. (Be careful not to do it so hard that you bend your phone). (3) Switch your phone to Flight Mode and back again to reinitialise the network search. (4) Repeat if necessary. This should re-seat the antenna and restore network connectivity. I had exhausted all other software options, and this is the only thing that actually worked for me! I've been doing this for a few weeks now and it consistently restores service to my iPhone 6. I still lose network every few days (sometimes multiple times in a single day), but this method restores it every time.