I do suspect you're doing it wrong, and that there's a simpler, more sensible option to this.
You have a local IP address scope (192.168.x.x) and a global IP address. Your dynamic domain links to the global IP address at your router. Any attempts at DNS would end up linking there. There's almost no way around it.
If its always the same remote computer, set up a VPN at the site with the cameras and connect through it - it would make the remote site part of the local site's lan. I'd go with openvpn here, but there's many options.
If its different remote computers you could probably use something like pagekite, which is essentially a reverse proxy.
If the cameras supported IPV6, which I doubt, that would work as well.