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 mfw_nrf9160_1.1.0.zip, 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 ? 

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