Sounds more like a wireless signal issue. Alot of times the TV itself acts like shield, blocking and interfering the signal. A simple solution might be a USB extender. I would try that first before messing around with the router settings.You can also find them for 5$ or so at walmart. Also, I assume you have the USB power cord connected to the Chromecast?
As for QoS, I guess I was wrong, this router uses Wireless QOS or "FasTrack Plus".See this manual for more info. Its designed to work automatically, so if its acting up, I would first try disabling it and see if the issue gets better. If it doesn't, or gets worst, then I would suggest these settings.
1. First you need to set up DHCP reservation for the Chromecast unit. See page 70 of the manual above for more info. You should see the devices name (what ever you named it during the Chromecast wizard) listed. Record what ever IP you assign to the device. I will call it 192.168.1.x going forward.
2. Then to set up FasTrack Plus see page 48 of manual above. You will want to use these settings;
Name: Chromecast (Doesn't matter)
Priority: 1
Protocol: Both. If this is not possible, make a seperate rule, one with TCP and one with UDP.
Local IP Range: 192.168.1.x through 192.168.1.x
Local Port Range: 80, 443
Remote IP Range: 0.0.0.0 - 255.255.255.255 (Unless you know Netflix IP range for your area)
Remote Port Range: 80, 443