Can you share the key parts of your code that are working with the cellular connection?
There is a known issue where the modem sometimes incorrectly reports that there is no sim card.
There is an option that’s carried over from the photon that allows you to connect without entering listening mode if there are no credentials (SIM card.)
Cellular.connect(WIFI_CONNECT_SKIP_LISTEN);
(We will change the name to NETWORK_CONNECT_SKIP_LISTEN in a future release to make it network neutral.)
I’m not sure why the modem stops thinking there is no sim card inserted, and I can’t say if it’s a transient condition that will subsequently fix itself.
@BDub knows the modem well.