Image: UbisoftOfficial screenshot from Ubisoft.
That's Puzzling asks you to unlock 15 Criptograma Chests, which is every one in Yara. Each chest holds unique gear, so the effort pays off before the trophy pops.
Find each chest, then track down the guerrilla sigils nearby to learn its two symbols. Set the lock to match and it opens. Work through the regions one at a time so you do not leave a lone chest behind.
Start with the single chest on Isla Santuario to learn how the puzzle works. In each region, mark the chests on your map before you go looking for sigils. The chime sound helps when a chest is tucked behind rocks or foliage.