This is a bit trickier because the firmware supplied by the manufacturer is very hard to configure compared to other modem routers, in any case I will try to help you.
First you need to force all your client to use a unique IP address example your laptop should have an IP address of 192.168.1.2 and your desktop 192.168.1.3 so on and so forth, this can be done by going into LAN settings and specifying an IP for a specific device.
Now go to your modem GUI and proceed to IPQoS settings.
IP QoS: Enabled
Default QoS: stream based
Source IP: 192.168.1.x (IP address of your target device)
Destination IP: 192.168.1.1 (IP address of your router OR a specific WAN IP address)
Netmask: 255.255.255.0 (depending on what IP address class your using
Port: 80 / 8080 (Web) Google other ports for other uses
Protocol: since your firmware doesnt support (both) option. Choose TCP and create the same for UDP.
Physical port: VAP0 (for all clients connecting to this MBSSID) so on...
Outbound Priority: p3(lowest priority) depends what you want to accomplish
TOS: depends what you want to accomplish
and repeat process until you add all your clients.
Make sure you "save" and "reboot" your modem for changes to take effect.