Try running a wireless sniffer at your problem site to determine conflicts on channel assignment. Note that non-WiFi 2.4GHz transmitters (wireless cams, security systems, etc.) may not show up in the results.
The last time I diagnosed wireless issues at a show, the WIU was broadcasting on channel 149 802.11n,ac, and channel 11 802.11b,g,n. When I logged on to the club's WIU LuCI interface, I briefly looked at the station mode tab ... IIRC, the channel configuration could be managed there.