If you did not see
/dev/ttyUSB0 in the device list, and you could not see anything matching your device, you will need to make sure the serial device is connected and supported by Linux. Then try the following:
If you do not know what your serial device is called, you can do the following.
ls /dev/tty* again. Did you notice something missing compared to the previous printout?
ls /dev/tty* again. Do you see a new device in the list?
dmesg to see if it is detected. In my case it is detected, and is named ttyUSB0:
sudo nano /boot/vmodem/vmodem.sh
serport=ttyUSB0 and update it to reflect the correct serial device.