@Aditya I just signed up to CNET for only one reason. To thank you
Follow Aditya's guideline and you should be able to save yourself from ending up with a brick phone or having to explain to that mobile service person why the hell we selected the USA band in the first place!
just make sure to do the following before you input those commands in the command window:
1) Make sure to install " Android ADB Interface" Driver for your mobile.
2) Make sure to install "Android Bootloader Interface" Driver (also called "Fastboot driver") for your mobile.
The "fastboot erase" commands will NOT work if you do not install the fastboot driver. In other words, if this driver is not present, the "fastboot erase modemst1/st2" commands will just keep waiting in an infinite loop. It will just say <waiting for any device> and will not proceed with deleting the modemst1 or modemst2 folders in our phone that were created when we stupidly selected the USA band
So make sure you do that. Below is a tutorial I stumbled upon, that would help you install the above 2 drivers. The tutorial is pretty long, overdrawn and the suggested software occupies a lot of space. But we can always uninstall these unwanted software (JDK and Android Studio) once we have executed the commands outlined by Aditya and as soon our phones are back to normal.
Tutorial --> How to Set Up ADB and Fastboot with Android SDK on Windows 7 & Windows 8
Note: During the fastboot driver installation process (page 3 of the tutorial), if you are unable to get into your phone's "fastboot mode" even after pressing and holding down combinations of volume and power buttons, just make sure you have the "ADB interface driver" mentioned above all set and that the command prompt command "adb services" correctly returns your device number as described in that tutorial. Then, just key in the first command alone, which is:
adb reboot bootloader
This command sends instructions to your phone that puts it in "fastboot mode" or "bootloader mode", which is essential for:
1) Installing the fastboot driver
2) Executing the "fastboot erase modemst1/st2" commands in the command prompt window.