These phones are pretty straightforward what I have found is that the antenna gets knocked loose very easily and maybe the new os is more finicky about signal strength. If you have dropped or bumped your phone it will pop off the antenna connector but stay connected enough for use but it will drop calls show no or very weak service signals.
Easy to pop off back to check antennas just resent.
There is a guide on fixing site.
May not solve your problem but yah gotta start somewhere.