It's been quite a long time since I've read the Mortal Engine Quartet, and I'm basing this answer off my memory of the books.
I think the main reason why static settlements near The Hunting Ground don't survive very long is because of the sheer overwhelming power at the disposal of traction cities. I can't think of many instances when a static settlement was consumed by a traction city, but there are a plethora of traction vs traction action scenarios. The thing is, a traction city already has a hard enough time fending off another of its own kind. A static settlement is basically a traction city disabled beyond motility; easy prey.
However, there are instances when a "static" city successfully fended off a traction city. When Airhaven needed repairs, it was settled down on an island, the surrounding body of water supposedly a good boundary for predator cities. Airhaven was attacked, however, by one of London's satellites cities, equipped to traverse the water. 308
If memory serves me correctly, Airhaven had the firepower to completely disable the attacking city while it traversing from water to island shore.
So, basically, massive cities like London (and that one city London destroyed with MEDUSA [can't remember the name]) are nearly unstoppable forces against static settlements—they are literally moving mountains of metal. However, if you're incoming predator is at an inherent disadvantage while approaching you (perhaps a steep incline, body of water, etc.) and you have the necessary armaments, a static settlement could hold it's own.
Edit: upon revisiting the question, I think the main challenge of a static settlement would be acquiring the resources to fight off what is effectively a hoard of metal mountains trying to eat you. The Shield Wall is made of the deck plates of fallen traction cities, and I imagine a static settlement near The Hunting Ground would employ something similar, using their fallen enemies as shields against future assailants. Regardless, a static settlement needs resources to keep up the perpetual fight (with all that metal, they make an attractive target), and I'm not quite sure how a static settlement would acquire those resources. I remember thinking about this while reading those books. I couldn't see why a static settlement couldn't/wouldn't employ motile cities to defend and fetch resources for the static city.
Anyway, those are my thoughts. I hope I was helpful! 46