Short update to my question. As far as I understand the problem isn't the pyserial library. It seems like the problem is how linux works with serial devices. Often the tty port itselfs don't allow more than 115kbits. So normally you can modify the port to support higher speeds. Some informations are here: https://discuss.96boards.org/t/linaro-ttymsm1-uart0-dmx-250000-baudrate/887
So I have to execute the following commands:
apt-get install setserial
setserial -a /dev/ttyUSB0 spd_cust
setserial -a /dev/ttyUSB0 divisor 96
stty cs8 -parenb cstopb -F /dev/ttyUSB0 38400
setserial -a /dev/ttyUSB0
But unfortunately setserial and stty are not availible in the repros. But I found them in some newer repros at openwrt. Next problem I have is, that setserial is not working. I get the following error:
root@Omega-11E7:~# setserial -a /dev/ttyUSB0 spd_cust
Cannot get serial info: Not a tty
Some additional infos I have found: https://www.downtowndougbrown.com/2013/11/linux-custom-serial-baud-rates/
But I don't understand how I can use this formations. I am not a C developer.
Hopefully someone can help me with this new informations.
Thanks a lot.