Arduino UNO HC-05 with stepper motor

Question

Im in trouble. I want to control stepper motor 28BYJ-48 with HC-05 on Arduino UNO. I'm using below algorithm, which I found on the net and adjusted to my needs. Im using RobotRemo android app. Steering element is the slinder in range [0;100]. The issue is: When I launch Arduino and connect HC-05, diod TX is blinking and HC-05 sending to microcontroler coordinates of slider. When I move the slider first time, motor works, but after this HC-05 dont send anymore coordinates of slider, it stopped refreshing.

Connection of HC-05:
HC-05  Arduino
VCC->5V 
GND->GND
TXD->RX
RXD->TX

Serial monitor:

20:11:10.678 -> s50
20:11:10.758 -> s50
20:11:10.828 -> s51
20:11:10.863 -> s51
20:11:10.946 -> s55
20:11:11.012 -> s55
20:11:11.058 -> s81   - when motor start working, refreshing is stopped

#include <Stepper.h>
 
#define STEPS 32

#define IN1 11
#define IN2 10
#define IN3 9
#define IN4 8


Stepper stepper(STEPS, IN4, IN2, IN3, IN1);
void setup() {
Serial.begin(9600);

}

char cmd[100];
byte cmdIndex;

void exeCmd() {
  
  if (cmd[0] == 's') {
    unsigned int val = atof(cmd +1);
    Serial.println(cmd);
    
    if( (val > 40) && (val < 60) )
    {
    digitalWrite(IN1, LOW);
    digitalWrite(IN2, LOW);
    digitalWrite(IN3, LOW);
    digitalWrite(IN4, LOW);
    }

    else
    {
    while (val >= 60)
    { 
    int speed_ = map(val, 60, 100, 5, 500);
    stepper.setSpeed(speed_);
 
    stepper.step(1);
    }
    while (val <= 40)
    {
    int speed_ = map(val, 40, 0, 5, 500);

    stepper.setSpeed(speed_);
 
    stepper.step(-1);
    }
    }
  }
}

void loop() {                      
  if (Serial.available())
  {
    char c = (char)Serial.read();
    delay(3);
    if (c == '\n') {
      cmd[cmdIndex] = 0;
      exeCmd();
      cmdIndex = 0;
    } else {
      cmd[cmdIndex] = c;
      if (cmdIndex < 99) cmdIndex++;
    }
  }
}