With every firmware so far that has been on the new OpenWRT release the WAN port on my wndr3700 and my tplink1043 have been picky. They work fine on 1.5.x before the openwrt change and they work fine on the stable 1.4.x releases.
If I try enough times the router (any of them, I have 5, don't use them at the same time) will end up picking up a useable ip and such from the WAN port. And a majority of the times this only works after the initial boot from a reset (using the actual reset button, not reset from the firmware options).
Again, I don't have this issue with any of the routers on earlier 1.5.x or the 1.4.x branch. Nor do I experience this with the routers on their manufacturer firmware.