NAT loopback on v380.57 and RT-AC68U

I've been trying to get NAT loopback working on my RT-AC68U, but have had no luck. I've tried:
- Both Merlin and Asus NAT Loopback
- Disabling NAT acceleration
- Disabling/enabling the firewall
- Disabling/enabling port forwarding.

I did a factory reset and two power cycles when flashing Merlin (as advised elsewhere on this forum), but I still can't access LAN resources (IP cams, web servers, etc) using external IP addresses.

Has anyone got NAT Loopback working on the RT-AC68U with Merlin...

NAT loopback on v380.57 and RT-AC68U