Riddle Me Fish is one of the hidden treasures in Far Cry: New Dawn. Just like with the previous missions of this type, you can learn about this one in Prosperity, in one of the captured outposts, or while you are exploring the world. The exact location of the mission is presented in the picture below:
Reach the place and collect the note that starts this mission. Enter the property and go further - you will see a burning bandit running out of a small barn. This is the only enemy in this area.
Defeat the bandit and then enter the barn. You find a manhole that leads to some kind of basement. There, you will find a small room with another note. From here, you can access a corridor that leads to the stash. To open the door, you have to activate the fish in the correct order. If you make a mistake, the room will get filled with fire. The correct order is - 2, 4, 6, 1, 3, 5.
The door is open - go inside and collect resources, ammo and perk points.