Just wanted to create a warning, this achievement can glitch, as it did to me. The problem, is I don't know what caused it to glitch and not unlock.
So when it glitched, I had freed all of Insula Fonte before doing the Missile Cowboy mission, which I believe is the last Act One main mission. My stat page also said that Fonte was fully liberated. I noticed it didn't pop at the time, and I assumed maybe I had to do all the Act One missions. Which I then did, and it still didn't unlock. Yet I have talked to a few other people who said they unlocked it before doing all the Act One missions, which just added to the confusion.
So I completely deleted my save and started over, and I wanted to list what I may have done differently, to insure it did unlock this time, which it did. First of all, I didn't leave Insula Fonte at all, I stayed within the borders of the starting islands. I had left them in my first playthough, to go get the jet at Cauda, which is part of Insula Dracon. I also made sure not to die the second I freed a settlement, as I believe I may have a couple times in my first run. And of course I did all the Act One missions before freeing all the settlements. Finally, I also made sure not to liberate any settlements(I did destroy some Chaos Objects while doing a main mission, but I never completely freed any settlements)while doing a main mission.
Just wanted to write something real quick to warn others this is capable of glitching. I wish I knew for sure why this glitched for me the first time, but sadly I don't really know, but maybe if you follow my steps it'll unlock just fine for you.