the reason is that the building have some very heavy wall and the 5ghz works fine on the corridor for example but.... not as expected behind the wall...
For example, good coverage on the coridor which have the ap mounted and not really on rooms which doen't not have ap installed.
The 2.4ghz signal is better on the rooms... and we need wifi on the rooms...
that why i would like to use only the 2.4ghz channel...
Actually the Android (Xperia) phone doesn't not roams as expected... and we've got couples of second of network cut...
By just force the ssid on 2.4ghz, the phone will stay on the band and do not jump between the both band and cut the network..
(works fine and Iphone or Nexus)
If you have some tips and tricks to improve this behaviour do not hesitate to share