I spent a day to figure out how to enable and make the DMZ working on virgin Hub3, finally I made it working and would share this here for everyone.
1) Even if the device which you want to put in DMZ (i.e PlayStation, Raspberry pi , firewall, etc) has a static IP , still you need to configure the device IP in Virgin Hub as a DHCP reservation. its stupid, but this is how it works.
So first, head to DHCP setting, add the MAC address and the IP of the device and reserve it.
2) Ensure the Firewall is enabled! check on Security -> Firewall and make sure it is enabled. you can turn off the other features such as detect scanning or drop fragments, but ensure Firewall is enabled. Without having the Firewall enabled IT WILL NOT WORK. imagine it as a iptables process that needs to be enabled for a destination NAT.
3) Configure the DMZ , set the IP address and apply. also keep in mind every time you just visit this page or the port forwarding it will tell you to apply the config although you haven't made any changed. (bad QA on software).