It was in the changelog I think.  WMM is required for 802.11n/ac

  @mackworth Yes, it was in that. But the question: why?

  @mackworth and why can't connect 802.11n/ac without wmm?

  @Daniel-nio Because its pretty fundamental for 802.11n and above.  Turning it off is probably not the solution to the problem you are having.

  @Daniel-nio You can only do Rate Limit on Omada Controller and if you limit the EAP speed to 54Mbps, then it will work on 802.11 b and no WMM enable. 

However like mackworth said, WMM may be not related to your original issue. Can you share more details about why you want to disable WMM?

  @Somnus Hy Somnus!

Our own wifi system have 30 EAP on 4 floors. The clients has a smart tv, internet radio etc.. so the three wifi channels with 20 Mhz are 70-80% utilization before I turned off the wmm. After that it is 30-40%. Plus I enabled the Minimum, Data Rate Control/ Disable CCK Rates + Send Beacons at 1 Mbps. The clients told me before that(and I also saw), connection lost problems in the oldest and newest devices too, after that it is work fine.