Yes, you can use this
router's LAN Ports as a WAN Port, In fact it can be configured to use 4 different wired ISP attached to all 4 LAN Ports simultaneously(depending upon the type of IP allocation done by ISP) and yet been able to access this router config page for management & all. All this with the stock firmware that comes with the router. I've used it with 2 lan ports (BSNL + Local WISP RJ45 coming from Roof) both working at same time with a simple local ip change in
windows e.g. 192.168.1.x-BSNL & 192.168.2.x-WISP).
Now coming to your scenario, You need to provide a bit more information about the type of configuration your ISP provides. For a start go to this routers config page 192.168.1.1 (admin/admin).
Under the advance config settings delete all the ATM interface and WAN service both.
If you are assigned a dynamic ip from your ISP then disable DHCP service under the LAN Settings in the router and choose automatic IP assigning in windows. Hook your RJ45 cable in any of the lan port and windows will pick the IP automatically. If the ISP is providing some Static IP (mostly ISP distributing in bridge mode) just enter those IP in windows IP details and check.
There are a couple of more different ways like setting up a Bridge WAN or a IP over ethernet mode under WAN settings, which can also be tried but this router even works without any config under WAN. It just works given correct IP details are setup at correct place. So do provide some more details like which type of ISP is it - Wireless ISP with a RJ45 coming from a Wireless device or a local cable guy providing service with private IP allocation e.g 172.16.x.x ip, 255.255.255.0 Subnet and 172.16.x.1 gateway server etc.
PS - My current setup is like this-
192.168.1.1 -
Wireless device provide by my ISP (this device is configured with a private static IP something like mentioned above. This device has DHCP service configured for 192.168.1.2 -192.168.1.254. A RJ45 lan cable coming from the device it going into one of the LAN ports of the router.
192.168.1.2 -
D-Link 2750u Router IP address (changed from default 192.168.1.1 under
Advanced config ->LAN) with
DHCP service off.
Now all my device gets automatic IP from the device 1 handling DHCP requests and Device 2 broadcasting wifi. Internet works just like breeze. And I am able to access both device config page at same time by accessing 192.168.1.1 & 192.168.1.2 respectively.
For some service provider it will be a bit different but its all about putting the right IP and gateway address at right place.
