If you have a locked internet modem and would like to use the SIM card of any network with it, you can easily get this done by unblocking the modem directly with a modem unlocking software or using a generated unlock code to decode the modem.
Huawei modems can easily be unlocked compared to ZTE modems. If you enter the imei number of your huawei modem into an huawei unlocker, the unlock code will be generated for you which you will use in decoding your modem. There are also some websites where you can generate modem unlock codes online.
Once you have the unlocking code, write it or save it somewhere. Then, insert ANOTHER SIM card (apart from the original network that originally works with it) into your modem. You will be prompted with a bar to fill in the Unlock code. (It usually gives 10 attempts). Just input the unlock code. That's All.
Thereafter, you will have to create new profiles for the networks you want to be using on the modem. If you are a Nigerian, you can check "Steps to take after unlocking your modem" for info on how to create internet settings for your modem.
If your modem is a ZTE modem e.g etisalat zte modem, you can check out this tutorial on Nairaland on how to use dccrap in unlocking zte modem. You will also find the download link of the dccrap software on the thread.
Below are snapshots of two different unlockers: