Hi sao.fujiyama,
Good day.
Are your firmware version currently on JP-80.30.96.221?
Please also help me to remove the SIM card, restart the device then re-insert the SIM card for a check.
I would suggest you can try to reset the network settings.
(settings> system> reset options> reset wifi, mobile & Bluetooth)
Does this problem appear if you use a sim card of another carrier?
Could you show me SIM status screenshot when you encounter this problem?
(settings> system > about phone > status > sim status)
Thank you.