How much bandwidth do you have, and what type of internet connection?
Are you running on such a small amount of bandwidth that any other devices streaming or downloading would cripple your internet is my concern. I have configured other routers with bandwidth priority and you need to know what your maximum bandwidth is and then go in and configure it to within the bandwidth that you have. So say you only had 3GB/s bandwidth and you had your router set up to give say 60% bandwidth to the computer and 40% to the other devices, with any other devices active it may through bandwidth allocation chop you directly to 60% when any other traffic is going on in which you have 1.8GB/s and the other devices are given 1.2GB. I had to find the balance with my internet connection when I dropped my bandwidth to save money when the broadband in my area jumped in price. I'm on 5/1 bandwidth for $45 a month which is the bare minimum that works and it only works for me when I have my router which I flashed with DD-WRT setup to allocate portions for priority of service for VoIP phone to be say 1Mb/s by 384kb/s, leaving computers and all other devices with 4Mb/s by 640kb/s so that the phone would not cut out when watching netflix or downloading or playing online games etc. While in games latency is not bad, usually less than 100ms.
Other thing to look into is I am not familiar with FritzBox, but there may be an Enable/Disable option in there that was overlooked in which you think its enabled for priority of service, but its actually shut off. In DD-WRT there is a check box that could easily be missed when setting up the priority of service. Also for priority of service I needed to know the MAC address of the devices that I wanted to give priority and set them up to have a DHCP reservation where they always got specific IP addresses issued to them. It took me about 3 tries to finally get the balance and configuration right with DD-WRT for priority of service and then when i got it working correctly I went in and wrote down all settings into my tech notebook where I keep important config stuff to look back at in the future when say the router needs a factory reset for some reason and I need to reconfigure it back to where it was working before.
If your unsure of your bandwidth you can shut off all other computers and streaming devices and run a bandwidth tester at
www.speedtest.net which will give you a download and upload speed. If this figure is nowheres where you should be for bandwidth you either have a device somewhere that is eating up bandwidth such as a neighbor connected to your wifi if you have wifi unsecured or easy to figure out password, or a problem with your service and should get your isp to perform a loopback test to see where the problem is located.
