In case anyone is here looking for the solution to the same problem, I found someone who seems to know what they're talking about giving you a fairly easy to understand rundown of what settings you need. One thing they haven't mentioned, probably because it should be pretty obvious (though it wasn't necessarily to me) is that the router should be in wireless access point mode, not ADSL router mode. Also, the quick setup options I spoke about above are only relevant to ADSL router mode so can't be used in this instance.
Click here for the thread
Hope this helps someone that's been struggling the same as me for a few days.
Much later edit:
As comment suggests, here's the relevant part of the thread. I would still advise you to try the link as he provides a bit of background but this should help out if it does go dead.
Turn off both wifi bands
Turn off DHCP (probably not necessary, but I feel it is a cleaner solution)
Leave the IP range at the default (192.168.1.1 to 192.168.1.256). This is the first of the three options.
Leave the IP address as 192.168.1.254
Plug the HH5 into the mains and into the telephone point. The blue light should eventually be shown indicating it has an internet connection.
This Netgear modem router has "everything" switched on i.e. DHCP, modem, wifi.
Set the IP address to 192.168.0.1 Note: this is a differnet subnet to the HH5 i.e. it is 192.168.0 rather than 192.168.1
The IP range for DHCP is set to 192.168.0.3 to 192.168.0.256. This is how it has always been set for my network. Note: the start IP address of 192.168.0.3 allows for 192.168.0.1 and 192.168.0.2 to be used for my two Netgear modem routers.
Now the clever bit:
Within the Internet settings, change the "Does your internet connection require a login?" to No. This had always been set to Yes on the DGND4000 along with the required account name and password. Setting it to No, removes the password field and provides additional fields to allow LAN details to be entered:
Set the Internet IP Addres as follows:
Use Static IP address
IP Address: 192.168.1.253 (this can be any IP address in the 192.168.1.* subnet as long as it isn't the same as the HH5! I chose a high address, not that it matters as none of my clients are on this subnet anyway). This essentially assigns a local IP address to your Internet connection, which will be used by the LAN to access the internet. Usually, the Internet IP address would be assigned by your ISP (either static or dynamic).
Gateway IP Address: 192.168.1.254 (this MUST be the IP address of the HH5)
Domain Name Server (DNS) Address
Use These DNS Servers
Primary DNS: 192.168.1.254 (again, this MUST be the IP address of the HH5)
Now use a network cable to connect the HH5 to the DGND4000. On the DGND4000 plug the cable into the WAN socket (NOT one of the 4 LAN sockets). Plug the other end of the cable into the LAN socket on the HH5 (NOT the WAN socket!).
Turn on the DGND4000 and hey presto internet access without a telephone cable being connected to it! In terms of lights on the front panel of the DGND4000 the only difference is that the DSL light is no longer on. The internet light should be green (it is getting internet access through the HH5 via the WAN socket) and all other lights are unafected by the config.