I have Arduino UNO with expansion board where installed BL module and LED. But now I would like to send some info via USB to work with LED. I have such code for python:

port = serial.Serial(port='...', baudrate=115200, timeout=.1)

print("waiting for arduino...")
line=b""
while not b"READY" in line:
    line = port.readline()

port.write(struct.pack('bb',  3, -10))
res = port.readline()
print(res)

for i in range(10):
    port.write(struct.pack('bb',  3, 10*i))
    res = port.readline()
    print(res)
    time.sleep(0.5)

port.write(struct.pack('bb', 3, 10))
res = port.readline()
print(res)

and for Arduino:

struct Position {
  int8_t freq, color;
};

int8_t color;
int8_t val = -1;  

void setup() {
  Serial.begin(115200);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.println("READY");
    pinMode(3, OUTPUT);
  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
}

void loop() {
  size_t nb;

  if (Serial.available()) {
     struct Position pos;
        nb = Serial.readBytes((char *)&pos, sizeof(struct Position));
        color = pos.color;
        val = pos.freq;
  }
}

the problem is that when I install this additional board over the Arduino I stop receiving data from the port. I understand that I close RX/TX ports and that’s why I can’t receive anything. But how to do it for communication with board with this expansion board?