What is the APN name that US Cellular gave you use? Is the SIM card activated? Have you ever used that particular SIM card on another cellular device, like a tablet or phone? Are you sure that there is sufficient US Cellular coverage in the area where you’re using the board? They don’t actually own any cell networks, they just piggy-back off other carriers service like AT&T and Verizon. And apparently Verizon doesn’t recognize the EnviroDIY SIM7080 as an allowed device on their network, so even if all of the other steps I’ve mentioned are okay regarding the APN and SIM card activation, if there’s only Verizon service in the area you’re trying to use the board, then it may not work. Do you know what cell providers have coverage in your area?