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Check if only 1 SIM is inserted in the device if your device is the dual SIM model.
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Check phone signal. In some cases, you are in a fairly closed or crowed place and cannot receive signals. Simply go to a high, ventilated place or near the center to get the best signal.
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Check if the operating system has been updated to the latest version: Settings → Software updates
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Check if only the JP Mobile APN profile is stored in the device. Other old profiles need to be deleted.
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Turn on and off "Airplane mode".
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Turn off the device, remove the SIM, gently wipe the chip with a soft, dry cloth, then reinstall the SIM. Reboot the device.