If you know please tell me
I know that your issue has absolutely nothing to do with communication between a Raspberry Pi and an Arduino, so I know that tacking it onto the end of this thread was wrong.
Besides, no one can help you without seeing your code, and some proof that the port is only communicating at 9600 baud despite being told to operate at a different speed.
I also know that you will have no success at trying to load a dll on the Arduino.