@dononeil 

I had the same problem but I use rate control, it works pretty well. Even if the signal is bad the speed should be at least 24 Mbps and that is good enough for most IoT devices.

if the speed is not 24Mbps, the device roams to another access point

  @MR.S Quite a number of my IoT devices negotiate 1-6Mbps speeds when they're idle and have good signal (~-60's).... Wouldn't forcing them to connect/roam at a higher speed like that just create a bunch more chatter on the network?

  @dononeil What does your LOG show you for the client device?  Does it roam and change AP's often?  Is it trying to connect at 5 GHz at -80 dBm, while 2.4 GHz is available at -70 dBm?  Some devices like TV streaming sticks seem to prefer a weaker 5GHz signal before they downgrade to 2.4 GHz.  You can also see logs of failed connection attempts.  The client device plays the largest role deciding how it connects or roams.  Locking to an AP doesn't solve, as the device attempts a connection, gets rejected, goes somewhere else, tries to come back...  watch the logs how that works out.  

Get really good with the logs.  They are not comprehensive.  There are search features to help cull the problem device and understand what is going on with it.  Share what you find into this thread.  I've found that some devices have poor roaming implementations.  See how a different device/manufacturer associates from that same location.  Use WiFiMan or similar to see what your phone does from that same location.  Analyzing RSSI and device associations would become more of an AP category, than under controller.  

Good luck!  

RF Dude

  @RF_Dude All of the devices that are having this problem are 2.4 GHz IOT only devices. They don't roam, they just connect to the first AP and stay there, no matter how many times you try to reconnect them. I never see a roaming message in the logs. The only way to get them to not connect to the distant AP is to physically shut that AP off so they are forced to connect to another one. It's obviously a problem with the roaming function, whether on the AP or on the device I can't say.