4.4 kitkat - Frequency band mofications for Samsung Galaxy S4

I recently purchased a used, unlocked Samsung Galaxy S4 SGH-I337M (for the particular model see here). The phones native bands don't include the 1700/2100 AWS required by the cell phone provider (WIND mobile). However, the phone is able to get both talk and data as desired. The previous owner told me that he modified the phone to allow these bands.

I'd like to know

1. Why was this possible? Is it because the phone has the bands but Samsung suppressed them or is the phone being hoodwinked into working without having both bands?


2. Whether there is a chance a firmware update or factory reset will undo this feature?

Background: I'm a bit familiar with unlocking AWS bands using qcn files

Answer

Ok then, let me begin by answering in order

1. 
   

   This is possible if the previous owner flashed their own hybrid modem onto their phone, which allows phones that are physically able to use a certain band (1700/2100) in your case, but the company does not allow it on their stock modem (for many different reasons). The hybrid modem basically overwrites the old modem that in short, says to the phone, "Hey, these are the only bands you can use", while a hybrid modem tells the phone, "Go crazy", and unlocks all or certain bands at a cost of battery life (more bands, more battery is needed to power it.)

   
   


2. 
   

   Most probably, yes. A factory will wipe out the hybrid modem and replace it with the stock. So only update if you have a copy of the hybrid modem, and know how to flash it. (ODIN)

   
   

I hope this answers it for you.