Power off the device
Remove the SIM card tray
Take out the SIM card carefully
Use a dry, soft microfiber cloth to wipe the SIM card
If needed, gently clean the gold contacts with a dry cotton swab
Use a small amount of isopropyl alcohol on the cloth or swab for stubborn dirt
Let the SIM card dry completely
Reinsert the SIM card into the tray
Put the tray back into the device
Power on the device and check the connection