US Cellular’s Galaxy S5 (model SM-S902L or R960 ) is often carrier-locked. Even with a valid SIM, the phone will reject non-US Cellular SIMs. If you’re trying to use a different carrier’s SIM, the “Invalid SIM Card” message is expected. You would need to request an unlock from US Cellular (must meet their unlock policy: paid off, account in good standing, etc.).
