If you have tried the paper trick, cleaning, and a factory reset, the internal SIM card reader on the LG G3’s motherboard may have failed. This is a known issue on older LG G3 units due to poor solder joints on the SIM connector.
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If physical inspection does not resolve the issue, the next logical step is to investigate software glitches. The LG G3’s operating system can sometimes "hang" on a previous hardware state, failing to recognize the insertion of a SIM card. The quickest way to reset the phone's hardware detection is a soft reset. This involves turning the phone off, removing the battery for a full sixty seconds, reinserting it, and powering the device back on. If the error persists, navigating to the "Network Operators" setting in the Wireless & Networks menu can help. Users should ensure that "U.S. Cellular" is selected as the network operator rather than "Automatic," as forcing the specific carrier selection can sometimes prompt the radio to reconnect. Additionally, checking for software updates via the Settings > About Phone menu is advisable, as older radio firmware can cause instability. If you have tried the paper trick, cleaning,