Doesn't work with tty made by CMUX

[insekt]

Hi.
I'm trying to use your code to get access to tty lines which is made by CMUX.
https://github.com/insekt/cmux
CMUX is creating 2 new virtual /dev/ttyGSM devices based on my physical ttyUSB0. ttyUSB0 is connected to GSM modem.
It works fine directly with ttyUSB0 but doesn't work with ttyGSM.

To be more specific I can send data to modem with your code via ttyGSM but I can't read reply from modem.