@Betterkeks: interesting, I had no idea about WMM (my router doesn’t have this option). But this is certainly worth investigating given the clear symptoms you reported. Let me know your router details and the firmware it’s running. The controller uses ESP8266, which is quite common in a lot of WiFi devices, particularly WiFi power sockets etc. I did a quick search of ESP8266 and WMM, and it seems to be an known issue:
https://github.com/esp8266/Arduino/issues/2795
I will be reading more about this and see if it can be solved in firmware.