you must have been having trouble with your modem by not been able to use another sim\netwok, here is the solution for you. if your modem is made from huawei or device you can unlock it with this application.
download here
How to use?
Just enter your IMEI number in the input box, click on the 'Calculate Code' button and your Unlock and Flash codes appear in the window to the left.
also after generating the code(note it) remove the modem, insert another sim and it will request for the code. so insert it.
Hope this will be helpful. please drop your comments