feat(modem): Add Delay after netif stop prior sending disconnect to CMUX virtual terminals

components/esp_modem/Kconfig

@@ -36,6 +36,12 @@ menu "esp-modem"
             The typical reason for failing SABM request without a delay is that
             some devices (SIM800) send MSC requests just after opening a new DLCI.
 
+    config ESP_MODEM_CMUX_DELAY_AFTER_NETIF_DISCONNECT
+        int "Delay in ms to wait before closing virtual terminal abd rigt after PPP disconnect(NETIF)"
+        default 0
+        help
+            Some devices might need a pause before sending disconnect command that closes
+            virtual terminal. This delay applies only in CMUX mode.
     config ESP_MODEM_CMUX_USE_SHORT_PAYLOADS_ONLY
         bool "CMUX to support only short payloads (<128 bytes)"
         default n

components/esp_modem/src/esp_modem_dce.cpp

@@ -160,6 +160,7 @@ bool DCE_Mode::set_unsafe(DTE *dte, ModuleIf *device, Netif &netif, modem_mode m
         if (mode == modem_mode::CMUX_MODE) {
             netif.stop();
             netif.wait_until_ppp_exits();
+            usleep(CONFIG_ESP_MODEM_CMUX_DELAY_AFTER_NETIF_DISCONNECT * 1'000);
             if (!dte->set_mode(modem_mode::COMMAND_MODE)) {
                 return false;
             }