It is a network issue 100% in your lan . If the box is not giving you a default gateway etc then the box is not announcing its prescense to your router and hence port forwarding etc will not happen correctly . The port is showing open when youve checked because you have setup port forwarding in the router hence the port is open. But if the router isnt seeing your receiver correctly with hostname , mac blah blah blah then some routers will not forward anything. Easiest option in your image is to disable dhcp and set your receiver on static ip and fill in all info yourself.
for example
your receiver ip : 192.168.1.2
netmask 255.255.255.0
default gateway 192.168.1.1 ( if that is your network range you have setup in your lan )
nameserver 192.168.1.1
set your box to the static ip for which you have setup port forwarding rules already in your router.
Any probs post back
regards
new2