You have a problem on disconnecting modem so the gsm network did not show again.
Please follow me:
1. remove modem
2. shut down pc
3. boot without modem
4. wait for the system to fully load
5. from terminal: sudo stop network-manager
check that system responds "network-manager stop/waiting"
6. from terminal: sudo killall modem-manager
you can check that nothing is running with: ps -A | grep -i manager
7. from terminal: sudo dmesg -c
8. NOW attach modem and WAIT 30 seconds
9. from terminal: dmesg
10. from terminal: lsusb
Check again terminal output to find the modem and /dev/ttyUSBx ports
I hope this article can help you.
This article from the http://ubuntuforums.org/showthread.php?t=1735496