Ok, I had my problem solved now by trying different steps. I will register it here because it may help other users in the future.
1 - When I first tried to unlock my Fido Samsung S7 to use another carrier, the unlock code provided by Fido didn't work. The device asked for a Master unlock code. After that, Fido provided me with a second unlock code given by Samsung (it took 3 days to arrive).
2 - Now, with the two codes in hand I finally succeed to unlock my mobile following these steps: 1) I put another SIM card (different carrier) on my phone and turned it off and on. 2) when it asked for the Master unlock code I inserted the second unlock code given by Samsung (via Fido) and it showed "Network unlock successful." and right way asked for a 3) SIM network unlock PIN, in which I used the first unlock code provided by Fido, 8 digits, and not the 4 numbers that the customer service had suggested. It worked!
I hope my explanation was clear. Maybe this case is unique, but it took in total 5 days to be solved, and I felt that Fido service wasn't properly informed to deal with that.
