The antenna may have become dislodged.
# While sitting down, turn your phone screen facing down in a horizontal position across your upper leg.
# 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).
# Switch your phone to Flight Mode and back again to reinitialise the network search.
# 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.