Connecting GSM Dongle to Intel Galileo-Gen 2
Try updating the firmware. I'm not sure if this affects also ACM type serial ports, but the initial version of the firmware was not even creating standard /dev/ttySx, rather some weird proprietary interface.
Luckily this was fixed later on. So, in case you are still running the FW that the board came with, chances are you are having these weird serial ports.