How to switch from "stream mode" to "characters mode" with STM32 Cellular?

[ABizi.1]

Hi everybody,

STM32 Cellular can use modem serial port in both "stream mode" to exchange data (LWIP module), or "characters mode" (CS module) to send AT commands.

I need to switch from "stream mode" to "characters mode". How can I do this?

Note 1: I need to do this switching  to query RSSI signal by AT commands (after modem getting working PPP connection). It should be already done by STM32 Cellular inside, but It doesn't. In particular, when funtion CST_polling_timer_callback is called, the state machine is always CST_MODEM_DATA_READY_STATE and never goes to CST_WAITING_FOR_SIGNAL_QUALITY_OK_STATE. 

Note 2: using x-cube-cellular 3.0.0