I am joseph from Delta State Nigeria, I'm a field tech, I was ask to go install at a customer end, my experience yesterday was funny, as I keep getting some error message while trying to attach to our AP less than 5KM.
Do the logs on the AP or SM give any hint as to WHY it's failing? You may need to enable all syslog levels under Configuration->System, make sure Info,Notices,Warnings,Errors,Alerts,Emergency are all checked to get the most out of logging.
The errors in the log should help explain. (maybe WPA2 with passphrase mismatch, maybe range limit exceeded, or client limit exceeded, etc)
I would suspect it's just an interference issue - at that signal level, some fairly strong interference could easily be enough to stop it from registering.
Thanks everyone that has made their own suggestion, I'll try all you have said I think that should help me in the feature and I must come back here to share my experience with you guy... Thanks everyone.... Love ya