Confusion on QoS by service and by device

[upsuper]

I have a D7000 (which is supposed to be R7000 + modem, so I marked R7000 in Model). I'm confused by how QoS works when I have both rule for service and for device.

For example, I have a rule to put service A in high priority queue, and I have a device X which I put in low priority queue. If device X uses service A, which queue would this connection be? High or low or something else?

In my case, I hope all traffic from device X is at low priority, because device X is always on, and it has enough time to do whatever it wants. But it seems my strategy above doesn't work. Traffic from device X still significantly affects the network, unlike the low priority "by service" rules which work well.

Is there any solution to make QoS work the way I want?

Thanks.

[upsuper]

Oh... hmmm... It seems D7000 has a much worse firmware than R7000. The latter has dynamic QoS while the former doesn't... I guess there is nothing I can do except waiting for a better firmware for D7000 or buying another router...