New iPhone SE SIM card issue -- it repeatedly says "No SIM"

Got my phone yesterday from AT&T by home delivery. It's running 14.3. Around 5 times the SIM card ceased to be recognized. The phone will work for minutes or hours, then I will have to either remove the SIM card or power down for the SIM to be again recognized. It has occurred while on a call and while not being used.

Is the problem with the phone, SIM card, or software? How to resolve?