No, I’m assuming they had a plain ol’ telephone land line and a modem dedicated to the camera system, the minimum spec for the desired function. No disrespect to the departed intended.
You don’t even need that much.
A tablet or smartphone (even without any service) is capable of running a “closed circuit” security system.
Cameras are connected to the tablets (local) wi-fi and video is stored directly on the tablet.
Kind of like how a baby monitor works.