Your description is far from clear!
It may be that a packet sniffer (etherpeek or similar) on a) the internal network and b) the outside will be required to see where the failure is occurring.
It may be that a port or ports in addition to port 80 need to be open on the firewall, or configured to forward to / from the DVR.
BTW, if the DVR is assigned the fixed address of 192.168.1.220, then it is foolish to include it in the DHCP range (192.168.1.100 - 221), although the likelihood of the address being assigned is very low!
SD