[masraum]

So normally, your modem only has a single device connected to it, and when it's just that one device, everything works fine? You can't have multiple devices connected to the modem (by itself)?

I think Rockfan and Mike are correct. The wifi router has a firewall that is blocking access (the cable modem doesn't have a firewall, so that's why it works fine). The router should have a way to allow connections from the Internet into your network (you'll need to know a little about it, probably details in the security system documentation or online). Then you can set that up.

Even if you turn the firewall off in the router, you'll still need to setup port forwarding.

If you can let us know the make model of the Security camera system (assuming the NVR is part of the system, if not then the NVR details are what we need) and the details of the router, then we could probably point out the exact configuration changes that you need to make.