Note 5 (SM-N920CD) Unlocked But show only one sim, how to solve?

One Note 5 (SM-N920CD) was brought to me and it happened that somebody have unlocked the phone and is like he flashed it with firmware of SM-N920C instead of SM-N920CD and is only showing one sim even when the two sims were put in the phone and if you put one sim in any of the sim slot it will show, showing that all the sim slot were working but if you put the two sims, one will be showing.

Do I need to flash it with SM-N920CD firmware before it will be working for the two sim or do I need to upgrade it from version 5.1.1 to version 6.0.1 for it to work?

Secondly, if is a case where I must flash it with SM-N920CD, where will I get the file because is not in the support area?

Thanks for your time and please help me out!