Has anyone here tried using the WAN port of a TP-Link TL-WR1043ND as a 5th LAN port?

Previously my setup was like this:
Internet >> Linksys AG241 ADSL2+ Modem >> Linksys WRT54GS

Both the AG241 and WRT54GS were on a 10.0.0* subnet, so devices plugged into each (or wireless devices) could see each other. (e.g. wireless client could talk to printer that was wired to the AG241)
The WRT54GS was running DD-WRT and the WAN port was used a 5th LAN port and plugged into 1 of the AG241 LAN ports.

Now I have a TP-Link TL-WR1043ND, and wanting to do the same thing, so wireless/wired clients on the TP-Link AP can talk to devices plugged into the AG241, on the same subnet, 10.0.0.*

Anyone know if this is possible with standard firmware?
I had a play around with the factory web interface, but keep getting this error:
Error code: 5008 WAN IP address and LAN IP address cannot be in a same subnet. Please input another IP address.

Last resort is to install DD-WRT or openWrt, but thought I would check if any fellow Geekzone users have any experience on this device?