send out error : Socket error: POLLHUP and the connection was close by the cloud

Hello, I come from China, there is a new thingy:91 board,I had set bluetooth gateways,added sim card,added a device nrf-352656100749665 , update certificates and the modem was updated, then ,I restart deivce, which can be finded the network of CHINA MOBILE through nrf connect of pc,  but the device is aways reboot ,  print out a error  < Socket error: POLLHUP> ,an connection was close by the cloud.

    I have some questions:

        1: Why the device reboot ?

        2:how to  correct errors and let the datas show on nrf cloud?


  3:  nRF52840 of thing:y91 no any Bluetooth signal be show in nRF connect app when which used the sample code of  "samples/usb/usb_uart_bridge" ,I also program the demo of thingy91_nrf52_usb_uart_bridge_2019-11-29_d3130d77.hex, but the singal of Bluetooth not show in list of   nRF connect app.

     Which code can be used for the nRF52840_PCA20035 ?