There is a mountain pass from RRC to Jacobstown's area, I think that's where they're suppose to be. (Or you can loot them from dead bodies.)
Only quest-spawned egg clutches seem to count. The WIki says otherwise, but I had 12 Mantis eggs and they didn't count towards the quest.
There's a path from Jacobstown leading to the nest marked by the quest waypoint. I think it's also possible to access it from the side of the Remnants bunker.