Sorry for getting impatient. Not only the QOS is broken, but I'm experiencing frequent disconnection of mydlink camera due to its hostname non retention (DHCP problem, unable to register IP-MAC address pairing. Maybe more than 25 devices including NAS and IP cameras with phones, PCs, tablets and smart TV are connected. But I have DIR-636L connected to it as WLAN access point extension with DIR-880L as DHCP server and PPPOE dialer.