The server is disconnected and "user_esp_platform_discon_cb" has no callback.

[456vv]

With non-rtos firmware, the server automatically disconnects the TCP connection, and "user_esp_platform_discon_cb" does not have a callback.

[pratik]

user_esp_platform_discon_cb is not a standard SDK function in the latest nonOS SDK. Can you please specify which version of SDK you are using?
Use espconn_regist_disconcb() to register disconnect callback function.

Or please refer to wifi_set_event_handler_cb example on page 80 of nonOS SDK API reference v.2.0.1.