To set up a DMZ to an Araknis router is straight forward.
If applicable, the first thing to do is set the ISP router to a range away from the range of the Araknis router. For example, if your ISP router, by default is on 192.168.1.1, you will need to change it as an Araknis router out of the box also uses this range.
Sky and Virgin routers use the 192.168.0.X range. So you should be ok to leave these as they are, but that depends whether you want your LAN behind the Araknis to also be on a 0 range.
Regardless, the ISP router must be on a different range from the Araknis router.
You only need to create a DMZ in the ISP router.
1. Set your ISP router to a range away from any potential LAN's/VLAN's, I usually use 192.168.70.X
2. Log into the ISP router and find the DMZ set up, usually under Security or Firewall settings.
3. Input an IP address in the DMZ that will be the IP address you will use as the Araknis WAN IP address..... NOT THE GATEWAY (this will always be 192.168.X.1 or 254
4. Save the config
5. You might want to disable firewall settings in the ISP router as this can also affect certain traffic traversing the WAN to the second router. The Araknis has a pretty good Firewall built-in
6. In WAN of the Araknis router, set the mode to Static
7. Set the Gateway IP to the IP address of the ISP router, 192.168.X.1 or 254, whichever you selected.
8. Set the IP Address as the IP of the DMZ address you put in the ISP router
9. Set your DNS options if required and press Apply
DONE
If you have a device behind the Araknis router that needs to be on a DMZ, such as a server, or another router for instance, in the Araknis router > Firewall, enable the DMZ option and input the IP of the device you want on the DMZ