There are 21 provinces in the game, so this trophy requires you to visit almost all of them.
*** Spoiler - click to reveal ***The 21st, hidden, Province is Golem Island. It’s located in the far NE of the map, and can be seen in game as a volcanic isle. Try to visit it and a swarm of drones will take you down.
I’ve not yet found whether this is included in the trophy, I suspect it may be used for the Raid, coming in a few days time. Anyone in the know, clue me in!
Easiest way to get this is to grab a chopper, and simply fly around! Provinces can be discovered even if you don’t land and walk around.