Opening a serial port using a prebuilt .so library

Question

I have searched around StackOverflow and found several related issues, but none that answers my direct problems.

I want to open a serial port on a specific Android device through an Android app.

The device contained an original apk with some functionality for the serial port.

I have reversed engineered the apk, found the correct .so file in system/lib64 folder on the device so I know how the old app uses this project.

What i know:

• The serial port uses Uart for communication
• The .so file is used in another application so it has the correct functionality.
• Reverse engineering let me see the basic functionality and names used for the functions.
• The device runs Android 10
• It runs a arm64-v8a processor
• The old .so library is added to folder jniLibs, found and loaded.

I do not have the orginal .c file and .h file used in the jni setup for the library.

I do not have any documentation of the serial device or the original device since the manufactorer did not provide this, so building a new native library could be hard.

The reversed engineered liboldnativelibrary.so files method is this: (jobject i have put in myself from gidhra)

jobject Java_com_the_old_serialportApp_SerialPort_open
                  (JNIEnv *penv,undefined8 param_2,jstring inputString,speed_t baudRate,
                  uint hiddenParam)

I have written this code

package com.myself.serialporttesting

    companion object {
        init {
            try {
                Log.i("Library", "Loading library")
                System.loadLibrary("oldnativelibrary")
                Log.i("Library", "Library loaded successfully")
            } catch (e: UnsatisfiedLinkError) {
                Log.i("Library", "Library not loaded")
            }
        }

        private external fun open(
            fileName: String,
            baudRate: Int,
            hiddenValue:  Int
        ): FileDescriptor
}

I get the error message: No implementation found for java.io.FileDescriptor com.hfad.serialporttesting.SerialPort$Companion.open(java.lang.String, int, int)

I believe it is a naming issue since my package is called something else. Is this true, and is it possible to work around that?