DD-WRT router and port fowarding to VM

[wombat 2]

I have AirVPN connected via router running DD-WRT . I want to port forward a service running in a Virtualbox NAT virtual machine.

 

Currently I'm forwarding the port in the Airvpn client area, on the router using iptables to the host machines local IP and finally in Virtualbox NAT port forwarding.

 

Checking via http://canyouseeme.org/ shows the port cannot be seen. I know the IPtables rules are correct as other port forwarded services are reachable on the host machine.

 

Is this because the service is behind two NAT's i.e double NAT? Any suggestions would be helpful.