As you have explained, it seems that the modem is company branded, meaning that you will not be able to use any other SIM card from another carrier to connect to the Internet.
Usually these companies do not allow other SIM cards to be used with their devices but in many countries there is a law which allows a client to pay a small fee to unlock their devices (mobile phones, TVs, DVD-Players, etc). Please visit support service and ask around about this matter. You will see that it is more comfortable of using this method than alter the firmware or the software.
