How to remount FS for writing (RW) from an android application? Or how else to write to a serial port like /dev/ttyS0?

Question

Android

I have been suffering for a long time trying to learn how to work with a COM port from an android application. In Windows and Linux, this is much easier to do thanks to JSSC, and there are almost no solutions for Android. usb-serial-for-android doesn't even find the list of ports, and android-serialport-api just like JSSC doesn't even start before Android 5 due to the fact that for some reason in the native module methods using "termios.h" are not recognized. Perhaps it is not in the stripped-down core of the anroid.
And so I decided to write everything from scratch, but even zero does not come out.
If the command in the shell "echo 1234 > /dev/ttyS0" works well, the port receives a string, then I can't work with the port as a file, it says that there is no access:
Reason: open failed: EACCES (Permission denied)
At the same time, the device is rooted, I can even read from / system (I can't write).

Answer 1

Rooted exactly? Generally speaking, Permission denied means the lack of rights to perform the operation...

Answer 2

/dev/ttyS0, all of a sudden, is not on any fs.