iPhone won't connect to carrier network

My wife's iPhone 7 is currently not connecting to our carrier network. It connects to WiFi but cannot send or receive calls.

It did this for about 6 hours in July then has worked fine since, until yesterday. She is visiting in the mid-west, our home is on the east coast. In July we were together in our vehicle and while she was not getting connection, my iPhone had no problem.

Could this be a SIM card issue?