I am a person who doesn't like to replace thing by buying new one.
SO, here i will solve your problem.
First of all, you need to know what kind chipset do you have for your USB modem, which you could use a utility at
it you run of the util.
It will tell you pretty much all the information about your modem and please post your output of "dmesg" or "lspci" which will be very helpful to troubleshoot or find the driver you need.