Trying to get NAT loopback working on RT-AC68U

I've been trying to get NAT loopback working on my RT-AC68U, but have had no luck. I'm running Merlin 378.56_2, but have also tried 380.57. I want to access IP cameras and web servers on my LAN using their WAN addresses, but despite enabling NAT Loopback I am unable to do so.

I am using the RT-AC68U as a DHCP server and gateway. I have my Sky Hub (ASDL modem/router) connected to the WAN port of the Asus, with the Asus configured as the DMZ on the Sky Hub.

When connected to a 3G network I...

Trying to get NAT loopback working on RT-AC68U