Consumer Cellular is an MVNO that either uses AT&T or T-Mobile as its carrier. Most users are on AT&T's network. According to this prepaid phone news article, if your SIM ID (ICCID) "starts with 890126 it's for T-Mobile, AT&T SIM IDs start with 890141."
iOS users: Go to Settings > General > About and look for the ICCID number.
Android users: Use the SIM Device Info app that displays the SIM ID as "SIM Serial Number."
So when you choose which carrier to use in an account or member profile, select AT&T instead of Consumer Cellular if you are on AT&T.
If you are on T-Mobile, we will soon issue an update to work with that.