Like Ecnerwal already said, this is not going to work. (with just the NVG510 alone)
The problem here is that you're getting a public ip from your modem. This means the modem itself does not act as a DHCP-server. The other method you linked to only works if there is another DHCP-server.
Normally the router/access-point (NVG510) would get an ip from the modem via its WAN-port after which you could use its internal DHCP-server to issue private ip ranges on the LAN-side.
But because this WAN-port is a DSL-port and does not work (correctly) as a normal WAN-port you can't use it. So you would need to connect the modem to a LAN-port and that port should get the public ip. But thats not possible because all the LAN-ports should get the same ip-range to be able to communicate with each other.
There are 3 possible solutions to this:
- You could use a computer to directly connect to the modem and share the internet via a second network-adapter. You can connect your NVG510 to that because your computer acts as a DHCP-server. Downside is that your computer always needs to be on.
- You could get your hands on a cheap broadband router which only acts as DHCP-server (no WiFi necessary because you could connect your NVG510 to it)
- You could buy a less-cheaper broadband router which also has wireless capabilities.