I had the same problem. My phone is a ZTE that was running an old system
version. The following link helped me.
https://support.hellomobile.com/why-wont-my-sim-card-work-with-my-zte-smartphone/
I just fixed this on my phone. I have a ZTE and the system was not up to
date. The following link helped me.
https://support.hellomobile.com/why-wont-my-sim-card-work-with-my-zte-smartphone/
