I think port forwarding rana boss, but you have to know kung unsa nga type of port (uPNP, UTP, UDP) and port number ang ginagamit sa DVR nimo (usually naa sa user manual) and set that sa modem/router. If naa DMZ mode then pwde ra sad but it would be risky since DMZ mode opens all ports.
It would have been alot easier to configure if IP camera imo gamit, then you can access the camera maskin asa ka like sa laptop or cellphone (even if transit ka, naa 3G or 4G connection) as long as internet access ky it uses a dedicated IP address.
Hope this helps.