I think I know what he means. I am in battlefield 1 and I did one dungeon, completing three quests. When I go over another bridge there is another dungeon but its locked and there doesn't seem to be any keys.
to unlock those you will need to find a battlefield key in act1 that says that levels 2 3 etc are unlocked.
the key you have found only unlocks the first dungeon.. thats why the others have `locked` on them