Thank you for taking the time to reply.
As per your suggestions, I had reset the router to the factory defaults before using it; there are no access rules or filters in place that would be blocking me from accessing this AP remotely. Also, we do not have any other hardware which could be contributing to this such as a firewall. This is an AP which we have setup in an annex of the main building for very occasional use. The only piece of equipment in this building is this AP/ADSL Modem and nothing else. I am also unable to ping the DIR-615 remotely, even after I enabled inbound pings on the WAN IP.
Something caught my attention while looking at this issue today. Both times when I tried to connect to the router remotely and could not, there were no clients connected to it. As mentioned above, it does not get used all that much and there is no dedicated PC connected to it, just the modem. When I did a VPN and or a remote desktop to another network I was able to remote to the D-Link page just fine, but I was connected to it at that time so I was the client. This afternoon, when I left the site, people were still connected to the AP and when I got home I was able to connect to it remotely absolutely fine, I tried this again in a few hours after everyone had left and could not.
Do you think for me to be able to access the WAN IP on port 8080 this AP need to have a minimum of 1 client connected to it? Is this normal? Can this be changed?
Thanks again for your help with this, it is very much appreciated.