@tim-hilt 

Its totally common for 2.4ghz to be flooded, in fact i would be suprised more if it wasnt.   5ghz is your best answer and will offer the fastest speeds.

Have you set your SSID for both 2.4 and 5ghz frequencies on the same SSID?   If so have you enabled Band Steering and set preference to allow devices to move to the faster 5ghz?  This is all done via the controller

  @Philbert Hello and thanks for your reply! I just checked and the SSIDs and passwords are the same for both APs. But I cant find the option to control Band Steering! Not in the controllers settings and not in the AP-device-settings. I also tried searching for the setting with the search-functionality but nothing was found. I also installed the iOS app, but didn't find the setting there.

How can I set Band Steering? Where do I have to look to find that setting?

  @tim-hilt 

Working from memory here as i dont have a controller near me..  however its not an option on the App, you need to access the controller via the GUI

On the controller, its under   SETTINGS  >  SITE  >   tick ADVANCED FEATURES and it will be in there.. hopefully that is correct! 

Once enabled advanced features you will see more tick boxes for band steering, roaming etc..  personally i enable all these and select PREFERE 5/6ghz

That hopefully will do it for you

  @Philbert Thanks for the reply! I found the settings exactly how you described. Now my advanced settings look like this (I do have clients that are 802.11k/v enabled, so enabling fast roaming makes sense for me too):

I haven't touched the beacon control yet, because I don't understand what the values are saying.

Now that band steering is enabled, I expected for example my iPhone to be connected to the 5 GHz band. But unfortunately this is not the case. I assume that I have to tweak one of the threshold-parameters, so that it switches to 5 GHz faster.

Can you explain how I could achieve this? Or is there maybe another mystery at stake?

  @tim-hilt 

You dont need to worry about beacon control, really no need to adjust any of that stuff.   Airtime Fairness I do tend to enable on 2.4ghz only as its really congested and that helps with capacity and timings for IOT devices.

In terms of the thresholds, you shouldnt need to do anything as this should just work now..  However try rebooting your APs to get them to pull the updated policy.    Ive also found on Apple devices that you sometimes need to remove and re-add the WiFi to get it to work.

  @Philbert I'm very thankful for all of your responses. However, I feel dumber and dumber the more stuff I try out. I'm sorry, but after doing exactly what you described (which I can really see how it should make a difference), my pie-chart for the clients used bands still looks like this:

Sometimes I see one client switching to 5 GHz, but after refreshing the webpage, it already went up to 100% 2.4GHz. Maybe it really is a hardware fault? I can't explain it to myself any other way. My devices always show 2 or 3 of 3 bars of signal strength.

  @tim-hilt 

There will always be a delay of 30 secs or so on the controller webpage so dont take this as gospel, however if you have the same SSID set for both 2.4 and 5ghz enabled, band steering enabled then this should be all you need to get this working.

In terms of signal strength, dont be using the bars.. they lie.  Refer to the controller and get the clients -db strength as its much more accurate.   Apple devices are really bad in that respect and tend be inaccurate in the bars meter.