LinkSys E4200v2 Wi-fi often disconnects


I updated my openWrt some time ago to 18-06. What I remember since I booked DAZN (like netflix but for sports): my devices disconnect very often from wi-fi.

I have no clue, why this happens, but it didn't with 16-06(?) I had before on my LinkSys E4200v2 / EA 4500.

Do you have any idea, why this could happen? Here my wireless configs:

config wifi-device 'radio0'
	option type 'mac80211'
	option hwmode '11g'
	option path 'mbus@f1000000/mbus@f1000000:pcie@82000000/pci0000:00/0000:00:01.0/0000:01:00.0'
	option legacy_rates '1'
	option country 'DE'
	option noscan '1'
	option htmode 'HT40'
	option dtim_period '3'
	option channel '11'
	option txpower '20'

config wifi-iface
	option device 'radio0'
	option mode 'ap'
	option ssid 'MY_WIFI'
	option network 'lan'
	option encryption 'psk2+ccmp'
	option key '**************************************************'

Do you have any clue, where this may come from?

Thanks and kind regards,