Each device model has different hardware and can not share firmware for that reason. The drivers and drivers for the specific hardaware of each model are included in the firmware.
If you install a ROM of another model on your device, there are certain things that do not work.
In your specific case we have a SM-J320M Duos and you want to put SM-J320FN. To start goodbye to DualSIM and may not even have a network. But you may have more different things, imagine that the screen is different, it may be black.
In summary: NEVER INSTALL A ROM OF ANOTHER MODEL