It depends if it uses PPP (kind of traditional modem) or works like NIC (Ethernet card).
From what you write, I assume it's closer to modem. If it uses PPP protocol, you need a kind of dialling (may not need to dial, but it's needed to create a ppp device). If you have the modem detected, you may use kppp (should be an option to set your IP) or create the file you need for it manually. The filename is /etc/ppp/pap-secrets. The format of the file is
yourlogin * yourpassword
To use it and connect I use the command below (I have a similar connection, but it uses /dev/ttyS1 instead of USB, you need to change the device name):
pppd/dev/ttyS1 115200 modem defaultroute lock noauth login myloginhere
(115200 is my conection speed).