davidea
2
vpn - have you managed to set the firewall ?
what is the output of ifconfig ? do you have a vpn interface? (tun0?)
what is the output of route -n command??
usb modem , do you mean umts/lte stick , or old usb adsl modem?
what is the usb id of the modem ? (you can see it with the command lsusb, after install usb-utils package, or if your computer have linux directly on it with lsusb command)