iPhone 6s says no SIM card from time to time. I don't do anything specific just randomly throughout the day my SIM card seems to not register in my phone. A hard reset gets the cell service back again. Only started happening this week not sure what's happening
SIM cards have a delicate tiny chip in them so they can be easily damaged. I would suggest replacing the SIM if it becomes a constant issue. If that does not fix the problem, then there may be a problem with your SIM tray/reader. If your iPhone is under 1 year since purchase, you can get it repaired/replaced at an Apple Store.