If your Android mobile device is not connected to the network then re-insert the sim or restart the device to reboot the network connection.
You may face this error due to a hardware issue in the sim card or there may be a technical software problem in your Android device. Also, configure the APN settings manually to remove the error
Read more troubleshooting ways to remove this error: https://ityug247.com/fix-mobile-network-state-disconnected-error/