Unlocked Samsung Galaxy S5 from Sprint won't save any APN settings I input. Why not?

Hello,

I have bought an unlocked samsung galaxy s5. It seems it is a phone from the USA, previously on Sprint network.
The mobile data is not working.
I am trying to input the APN for my network here in Brazil (Vivo).
Every time I input the APN settings, and select 'save', nothing happens and the APN list is still empty. I have clicked 'reset to default' already. The list is empty - there are no pre-existing APNs.
I read somewhere else that it could be that the MCC or MNC code could be causing a problem, as the phone may think it is still in the USA. I don't know what that means.

Any help would be much appreciated!

Thanks