The Motorola SB6121 should be just a modem only and not a router, so that shouldn't be the problem.
So lets take a look at a few things.
Post 2 screen shots for me.
1. A screen shot of the port forwarding page in the router
2. A screen shot of the network setup in the DVR.
My port tester is only for testing to your computer, so we wont use that here.
Also make sure there are no port triggering rules enabled and DMZ is off in the router as well.
Lets go from there