Criptograma Chests are new to the Far Cry series and require you to find two Criptograma Charts to unlock them. One of these chests is located near the town of Ruben in the Barrial Region. Ruben is located in the middle of the region on the coast. South of the town, you will find the chest along the highway. Take a look at the map below for reference.
Once you reach the chest, you will see an arrow painted on the nearby cliff face. Follow the path along the cliff until you reach a warped dock that is falling apart. In front of it is an orange net holding back some rocks. Shoot the net a few times to break it and make the rocks fall, revealing a cave.
Head inside the cave and make a right. You will come across an area you can stand. There is a broken piece of wood here that has the first Criptograma Chart on it.
Follow the staircase in the cave and head out of the exit at the top of the hill. You will come out near the chest. Follow the road down the hill to spot a small shack with an arrow on it pointing to the water. Go down to the water to where there is a long dock. Check under the dock to find the second Criptograma Chart.
Now that you have both charts, you can head back up the hill and unlock the chest.