This is pretty common for every 1/10 Iphones. I always review the following.
# Check for Carrier Updates, this is typically done with the sim and wi-fi connected.
# Check for iOS Update
# Test for Signal, if fail proceed to next steps.
# Look for APN settings on the phone and remove it, this can cause a signal interruption. I believe it's in Settings>General> APN. This does not appear in all phones and this varies on where the phone was purchased from. Some APNs can be found in Cellular > Cellular Data Options > APN/Cellular Data Network. Just look around to see if one exists.
# Test for Signal, if fail proceed to next steps.
# Manually search for a Network in Settings > Carrier (turn off automatic) see if you can manually search for the tower of your carrier.
# Test another Sim Card to ensure it's not a hardware issue. Frankly, some phones, primarily samsung, gets picky about the sim type you use. I'm not familiar with Sim Cards and how they function, I just know that if one type doesn't work try another type.
Hope this helps.