Band steering, DAWN, 802.11v help needed

I have a router Xiaomi Mi Router 3G v2 with OpenWrt 21.02.0-rc3 and I set up 2.4 and 5 GHz networds under one SSID. When 5 Ghz signal is lost, devices would fallback to 2.4, but when 5 GHz is back, devices are not reconnecting to 5 GHz. I tried DAWN, it's not working at all. I enabled all Evaluate options in luci-app-dawn, no difference. When I add option bss_transition '0' in /etc/config/network wifi is disappering completely. I removed wpad-basic-wolfssl and installed wpad-wolfssl, no result. Some devices after disabling and enabling wifi are connecting to 2.4 GHz network even back-to-back to router. 802.11k and 802.11r (Over-the-Air) should be working, because I didn't noticed any delay when switcing to 2.4 Ghz. How I can fix it?