I remember having a similar issue with a wd my net n750 and openwrt bb firmware using luci web gui. Basically I had to force the 40mhz channel width for 5Ghz via command line. Also I've only ran the n750 in AP mode not client bridge; do you notice the same issue in AP mode? In any case you should be able to force 40mhz channel by editing /etc/config/wireless. For AP mode this my 5Ghz config (ispyisail latest 1.7.x release [3/26]):
config wifi-device 'radio1'
option type 'mac80211'
option channel '36'
option hwmode '11a'
option path 'pci0000:00/0000:00:00.0'
option htmode 'HT40+'
HT20 (single 20MHz channel),
HT40- (2x 20MHz channels, primary/control channel is upper, secondary channel is below)
HT40+ (2x 20MHz channels, primary/control channel is lower, secondary channel is above).