This is how I unlock my newly bought Mtn fast link modem without stress.
When you buy a new 3G internet modem from a service provider, the modem will be locked i.e other networks won’t work when you insert another Sim card different from the carrier network you will be asked to provide your unlock code because it has been locked by the network provider. The unlock code allows you to bypass the mode modem/network dongle for you to be able to use other networks on it.

How To Get Started
You need to get your unlock code, and how do you do that? I will show you how I generated my code.
this can be done with a software called Universal Master Unlocker. Master Unlocker is a calculator built to calculate modem serial number and generate the required unlock code which you can input on your Huawei to bypass it locked network.
You can get the master Unlocker software HERE for free installation is not required just download and paste it to your desktop double-click the app and it automatically runs just like your calculator.

At the back of your modem, you will see your IMEI number it’s a 15 digit number (356181043428463) now back to your Universal Master code application click on the “Huawei” tap delete the default IMEI on the application, and input your IMEI ( the 15 digits you found at the back of your modem) and click on calculate. Your result immediately display the app with different codes and write-ups copy the unlock code to your clipboard or write it down

Insert the new sim you want to your on your Mtn fast link 3G Huawei modem i.e if you have Mtn sim inside your modem all you need to do is replace it with any of these sims Glo, Airtel, and 9mobile.
connect your modem to your pc immediately your Mtn fast-link app is fully loaded on your pc a message will display on your screen requesting for your “UNLOCK CODE” paste/input the code you generated from your Universal master-code and click on the “OK” button
your modem has been successfully unlocked.
How To Configure Your Modem To Make It Work With Other Network
After creating your APN ( below it is the configuration settings ) then click ok. Use the configuration settings below for your preferred network.
Glo Access Point Settings
Profile name: Glo NG
click on static
Apn: gloflat ( no space )
Username: flat
Password: flat
Airtel Access Point Settings
Profile name: Airtel NG
click on static
Apn: internet.ng.zain.com
Username: internet
Password: internet
Mtn Access Point Settings
Profile name: Mtn NG
click on static
Username; web
Password: web
Etisalat( 9mobile ) Access point Settings
Profile name: Etisalat NG
click on static
Username: wap
Password: wap
You are good to go surf any web page of your choice on the internet using any of the above service providers in Nigeria
NOTE: This same process works on any Huawei 3G modem, not just Mtn alone.
Download the Universal Modem Unlocker Here
See How To Download From Pan.baidu.com OR Create baidu account in Nigeria

Quite informative