I personally unlocked this in Endless mode (Serenity, Easy with settlements turned ON). The achievement doesn't state that it has to be endless of the main scenario so I believe either should work.
After you have made contact with at least two settlements you will have to raise your relations with them to ideally be Loyal as every now and again a settlement will ask for aid, which you can either choose to help yourself OR send another settlement to help them instead. The aiding settlement will need to be loyal to you to allow you to select this option, if they aren't then you can only choose to help them yourself.
Once you've clicked to send the other settlement the achievement should pop straight away.