When you reach Ascuas Island you will eventually activate a quest where you need to fix the transmitter. To activate the transmitter you need two items.
1. Satellite Crank
(in the safe house on the eastern part of Ascuas Island)
2. The Transmitter Coordinates
(which are in a shack on the beach in the southern part of the island or just to the west of it, mine was at the right circle)
Once you collected both items go north and fix and activate the transmitter. Once activated the achievement will unlock.Note/tip:
At the transmitter you will have to fight two giant abomination type zombies. Come prepared to the fight. Bring explosives like arrows. It will take them down easy.