Edit: Some of the quests in this DLC glitch and may need to be replayed. I'm sorry if this happens to you, that's awful. But please don't downvote my solution because I don't know how to fix all glitches.
5th and final main mission. Will unlock after going through the Crater's Edge map, then returning to Vestige and collecting your reward.
goodgamez has a great walkthrough of just the main missions on Youtube. All credit to them.
This quest starts around 1:52:00 and the achievement would pop at the end of the video.