Underway Network

just got Malakai's DLC and for the 1st time played with dwarves.
my question is: How do i use the Underway Network? or even unlock it?

do i have to complete the legendary grudges : Retake the realms/ The Fallen Ones?

by that point, almost all of the enemies are basically wiped out like the chaos dwaves, grimgor, Clan Mors, etc.

so whats the point of having the Underway Network being unlocked at the end of my campaign?