The cellular towers look like satellite dishes on the maps. If they have a small yellow exclamation mark icon on the edge of it, that means that there is a mission available at it. Proceed to the tower.
When you arrive at the tower, search around the base for an electric box that should be flashing yellow. When you walk up to it and hit Y you should receive a phone call.
When you complete the phone call, there should now be a red target marker on the map. Proceed there and destroy the target it marks to finish the mission.
Continue taking missions from towers whenever there is one available until the achievement unlocks. Make sure you do them as soon as they are available. When you head to the south, you'll no longer be able to do the ones in the north.