I've notice odd behaviour with the OpenWRT firmware and "N Only" wireless setting on the 5Ghz. For OpenWRT I've been recommending that users not use the "N Only" but instead "A/N Only".
Try "A/N Only" it also broadcast wireless "N" and you know your connection is "N" because the rate would be higher that 54mbit/s.
Make sure you have your channel width settings to Auto.
WMM support must be enabled for wireless "N" to work at all on this router.
Keep Media Prioritization disabled.