Access to serial port on Android device - C++ or Other Android Development Languages

Hi Guys,
Currently I am working on an application build in Unity that controls Numato Lab USBGPIO8 module (it will simply turn on and off channels on gpio when conditions in a separate script in app would be met). Whole project is based on ARFoundation.
Opening and controlling serial port using Unity editor and PC works fine (my PC uses COM4 port). However, when building on Android I am getting an "IOException error: permission denied" when script tries to open serial port (gpio is now connected to smartphone through usb cable):
IOException: Permission denied
System.IO.Ports.SerialPortStream.ThrowIOException () (at :0) System.IO.Ports.SerialPortStream..ctor (System.String portName, System.Int32 baudRate, System.Int32 dataBits, System.IO.Ports.Parity parity, System.IO.Ports.StopBits stopBits, System.Boolean dtrEnable, System.Boolean rtsEnable, System.IO.Ports.Handshake handshake, System.Int32 readTimeout, System.Int32 writeTimeout, System.Int32 readBufferSize, System.Int32 writeBufferSize) (at :0) (wrapper remoting-invoke-with-check) System.IO.Ports.SerialPortStream..ctor(string,int,int,System.IO.Ports.Parity,System.IO.Ports.StopBits,bool,bool,System.IO.Ports.Handshake,int,int,int,int) System.IO.Ports.SerialPort.Open () (at :0) (wrapper remoting-invoke-with-check) System.IO.Ports.SerialPort.Open() SerialConnector.OpenPort () (at :0) SerialConnector.Start () (at :0)
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USB Device Info app shows the device path is /dev/bus/usb/001/002, so I use this as a port adress in the script:
C#:
using UnityEngine;
using System.IO.Ports;
using System.Threading;
public class SerialConnector : MonoBehaviour
{
SerialPort serialPort = new SerialPort("/dev/bus/usb/001/002");
//COM4 in unity editor pc
void Start()
{
OpenPort();
GPIOSetChannelOn(1);
}
private void OpenPort()
{
Debug.Log("Opening port");
if (!serialPort.IsOpen)
serialPort.Open();
serialPort.BaudRate = 19200;
}
private void GPIOSetChannelOn(int channel)
{
...
}
private void GPIOSetChannelOff(int channel)
{
...
}
}
I have added a line to manifest file, that allows external read write using custom AndroidManifest.xml file in Assets/Plugins/Android folder:
JavaScript:
<?xml version="1.0" encoding="UTF-8"?>
<manifest xmlns:tools="http://schemas.android.com/tools" package="com.unity3d.player" xmlns:android="http://schemas.android.com/apk/res/android" tools:node="merge">
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<application>
<activity android:theme="@style/UnityThemeSelector" android:name="com.unity3d.player.UnityPlayerActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN"/>
<category android:name="android.intent.category.LAUNCHER"/>
</intent-filter>
<meta-data android:name="unityplayer.UnityActivity" android:value="true"/>
</activity>
</application>
</manifest>
Despite these steps, the problem persists. Is there a way to resolve this issue without rooting my smartphone?