Frostback Mountains is a small, quite linear location, where you start the game and complete the prologue. Since the Mountains are connected with the game's storyline, as soon as during your first visit to the location, you explore quite a portion of it, although in some of the locations, you will be able to decide whether you want to stay there for a little longer, or progress the storyline.
The way to unlock this location: Automatically, after you start the game.
Recommended experience: You get to the Frostback Mountains at the beginning of the game so, you do not need to bother about leveling up.
1) As early as during the prologue, you will have to decide on your route. As a result, you may visit two different area s in the Frostbacks. Fortunately, you do not need to worry about what to do here, because in this location, there are no treasures to obtain and, as a result, you miss nothing.
2) Frostback Mountains do not appear on your map after you complete the Prologue, which does not mean that you cannot return here. You can visit the location later into the game, i.e. after you pick the access to this location at the Haven (the Haven chapter - M2 map). This becomes available after you start the Haven besiegement main quest.