What version is this for? PC or PS4. I have never seen this on the PC but have just read up about it.
Where To Find Splinter Cell’s Costume
Note: There will be a few hostiles to deal with inside the plane wreckage.
There are a total of seven expeditions and if you go through them you’ll get an expedition which requires players to track down Sam Fisher’s outfit from the Splinter Cell series. This is a relatively easy set of challenges for instance, you will need to head towards the government plane wreck which will be in two pieces. Reach the tail of the plane and head inside where you will need to locate a laptop.
Interact with the laptop which will allow a door compartment upstairs. Inside that compartment will be Sam Fisher’s outfit. Once you obtain it you’ll unlock the Paladin trophy. Likewise, the outfit becomes an optional set of clothing for the protagonist to wear.
I did exactly as you said and already have the outfit. The achievement just didn't unlock. I went back and tried the same thing multiple times but it still doesn't unlock. I have all achievements for New Dawn except this one and "The End of Eden." It seems the story progress bug happens to other people.