A. First, ensure that the SIM card is properly inserted into your device. Remove the SIM card and reinsert it, making sure it clicks into place securely. If the issue persists, try testing the SIM card in another compatible device to determine if the problem lies with the card or the device itself.
B. If the SIM card is still not functioning, perform a factory reset on your device as a last resort. Before doing this, back up your data. To reset, go to Settings > System > Reset options > Erase all data (factory reset). After the reset, reinsert the SIM card and check if it works.