Is the WAN IP address in the status page a public or private IP address? It should public IP.
If the IP address is not static, you can set up DDNS to bind the WAN IP and a domain together, then you can access with the domain. WAN IP address will be updated to the DDNS website if it changes.
http://www.tp-link.com/en/faq-1215.html

flying snow wrote

Is the WAN IP address in the status page a public or private IP address? It should public IP.
If the IP address is not static, you can set up DDNS to bind the WAN IP and a domain together, then you can access with the domain. WAN IP address will be updated to the DDNS website if it changes.
http://www.tp-link.com/en/faq-1215.html

Thanks for the reply.
The WAN IP showing in the status page is a public IP.
I have setup DDNS which updates fine, but it updates to the IP matching whatismyip.org, not the IP shown in the status page!
So when outside my network, if I browse to the DDNS domain I cannot get access.
Reading around the subject it appears that this is due to carrier grade NAT.
Does anyone know how to get around this?

If the WAN IPon the status page mismathc the IP on DDNS website, please test which one is your current Internet IP address. When connecting on VR400, open the google and just search for "my IP address", it will show up your current IP address.
If the IP from google search is the same as the WAN IP in status page, it should be the DDNS which is not updating the IP.
If it is the VR400 not updates the IP, try to reboot it and check if the WAN IP will update or not. You can also check if the VR400 is running the latest firmare, then upgrade it if not.