And after AAAALLL of that, I just changed my plan.
Same closet will contain everything: Cablemodem, Router, NVR all hardwired together. I'll just run ISP's coax cable to the closet through the attic. The computer can stay where it is, using WiFi instead of a hard connection.
I mean... Duh.
Somehow I was thinking the cablemodem and router has to stay where I asked the ISP to put it -- Just because I wanted my computer there.
Sorry, I've been deliberately wasting your time. Joke's on everyone. Including me. Ha ha.
Thanks everyone. Maybe this thread will still help someone.