This used to be simple on the old pre-7.01 firmware, I have found this option missing, in fact you seem to have to change an option to see more options: -
Goto Network/WAN, under interfaces click edit on WAN, it will likely be set to DHCP, change it to anything else then back to DHCP and suddenly Physical settings will appear, then you can try each of the interface options one at a time save & apply, then on the above menu save and apply again to see if it works.
I actually gave up on this as it never seemed to work, but I dont think I tried all interface options or the bridge option which I would presume is required, maybe I did, but I didnt get it going.
To FIX it I ended up upgrading one of my other sites with version RUT2XX_R_00.01 to RUT2_R_00.01.2 the same as the latest I had on the problematic devices (yes this was on 2 devices same issue), then I backed up the other sites firmware and restored it on the new units and they are now WAN=LAN working.
But if I then goto the WAN settings/edit to see what is the correct settings it doesnt show, so clearly a BAD bug.
Same with Wireless setup, the option to change the CHANNEL is GONE!!!!
Their are a lot of bugs on v7 but I have been using the now discontinued RUT2XX_R_00.01 on several sites, with OpenVPN and they have been bullet proof to date, but you will not find that version!
Maybe an admin can comment on these issues?