I found this notice in syslog :
20/40 MHz operation not permitted on channel
- as i understand this is due presence of many other 2,4G AP in range (over 30 AP around)
- also seems when the channels are picked (primary/secondary), sum of both should not be over 15.
- i found solution , using
noscan = '1'in wireless uci config in device section (but they note that this is against regulatory rules), tried and it is working (at least no notice in syslog).
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).
HT40 (2x 20Mz channels, auto selection of upper or lower secondary channel on versions 14.07 and above).
I was using HT40 without any issues till today (with auto for channels). Maybe HT40- might help to keep the sum of channels under 15 (will try tomorrow). For now i am using HT20.