Okay, so here's an update on what we've tried.
We got it working on the router intermittently, we could see and access it, then suddenly it would stop working.
When we tried using the crossover it worked flawlessly and continuously, unlike with the router connection.
We do need to port forward because we want WAN access to the DVR, but since we hadn't been able to access it locally (LAN) I needed to concentrate on that first.
What would make a network connection work sometimes and other times not? Apparently sometimes it'd start working then stop, then not work at all for an hour no matter how many times we restarted everything, then suddenly once it'd work again.