Scheduler - Serial, Timer ( interrupt priority)- help needed

Hi, I have a simple application with logic is:
- every 3 or 20 seconds send some data via modem (based on AT commands),
- sending and receiving data via serial port to and from the modem,

Application:
I)
When we take MCU ON the application each 3s is asking about the range (till range == true). After then the timer is switching TIME_INTERVAL from 3s to for example each 20s
II)
If we have a range app. is sending in each timer interrupt some short data.

Problem description:
I'm using two interrupt handlers, first is from the timer and second is from a serial port. When I wrote the code application was hang up each time during sending the data. By the trial-and-error method, I found that if I will change in NRF_SERIAL_DRV_UART_CONFIG_DEF()
UART_DEFAULT_CONFIG_IRQ_PRIORITY) to 1
All start works fine.
I spoke with my friend and he tells me that I should use scheduler and maybe then such situation shouldn't have a place.
I put the code from nrf_serial_evt_handler() to scheduler but the problem is the same:
if UART_DEFAULT_CONFIG_IRQ_PRIORITY then an application is hanging but when I change the prior to 1 start works fine.

Question:
How it should look like properly of using the scheduler in my case? and what I'm doing wrong?

Code parts:

int main(void)
{
	ret_code_t ret;
	/*Initialization, peripheral configuration.*/
	board_system_init();
	/*Application part*/
	bc95_rst();
	application_timers_start();
	/*Say hello*/
    ret = nrf_serial_write(&serial_uart, "Sensor is ready.\r\n", strlen("NBIoT sensor ready.\r\n"), NULL, NRF_SERIAL_MAX_TIMEOUT);
    APP_ERROR_CHECK(ret);

    while (true)
    {
    	app_sched_execute();
    	CheckBuffer();
    }
}

void nrf_serial_evt_handler(struct nrf_serial_s const * p_serial, nrf_serial_event_t event){
	if(event == NRF_SERIAL_EVENT_RX_DATA){
		/*if rx any char - put it to array*/
		app_sched_event_put(NULL, 0, serial_scheduler_event_hanler);
	}else if (event == NRF_SERIAL_EVENT_TX_DONE) {
		/*invert led if something was send via serial to modem*/
		bsp_board_led_invert(3);
	}
}

void app_timer_timeout_handler(void * p_context){

	ret_code_t err_code, ret;

	app_timer_pause();

	if (rangeNB == false) {
		rangeReady();
	}else if (rangeNB == true && lng_int_set == false) {
		/*If we have range we can change time interval*/
		app_timer_stop(m_app_timer_id);
		err_code = app_timer_start(m_app_timer_id, TIMER_INTERVAL_20S , NULL);
		APP_ERROR_CHECK(err_code);
		app_timer_pause();
		lng_int_set = true;
		ret = nrf_serial_write(&serial_uart, "changing time interval to 20s\n", strlen("changing time interval to 20s\n"), NULL, 0);
		APP_ERROR_CHECK(ret);
		bsp_board_led_on(1);																		/*Device have a range*/
	}else if (rangeNB == true && lng_int_set == true) {
		/*
		 * Make measurment and send it -
		 * - not elegant construction because we are in interrupt handler (try move it to scheduler)
		 * */
	    char tx_mesg_open[] = "AT+NSOCR=DGRAM,17,5000,0\r\n";
    	char tx_mesg_udp[] = "AT+NSOST=0,000.000.000.000,5000,4,64757061\r\n";
	    char tx_mesg_close[] = "AT+NSOCL=0\r\n";
    	
	    (void)nrf_serial_write(&serial_uart, tx_mesg_open, strlen(tx_mesg_open), NULL, 0);
	    nrf_delay_ms(50);

	    (void)nrf_serial_write(&serial_uart, tx_mesg_udp, strlen(tx_mesg_udp), NULL, 0);
	    nrf_delay_ms(50);

	    (void)nrf_serial_write(&serial_uart, tx_mesg_close, strlen(tx_mesg_close), NULL, 0);
	    nrf_delay_ms(50);
	}

	app_timer_resume();

}

Logs from serial port:

Board to modem> [13:41:54:219] Sensor is ready.␍␊

Board to modem> [13:42:00:206] AT+CSQ␍␊

   Modem to board> [13:42:00:228] +CSQ:21,99␍␊

   Modem to board> [13:42:00:228] OK␍␊

Board to modem> [13:42:03:229] changing time interval to 20s␊

Board to modem> [13:42:23:199]

//After print "A" mcu  hangs