I have connected a FritzBox 7490 to a HG659b wirelessly so ok so far - but want to give the FritzBox priority for streaming over other devices connected to the HG659b WLAN?
Still not possible.
The only way to give traffic priority over WiFi is by way of DSCP tagging which maps to WMM across the WiFi connection at layer 2. This is at an individual application level where you can set tagging (ie a VoIP softphone) or by using hardware with capabilities well beyond what you have to remap all traffic to/from individual IP addresses to DSCP.
What are you trying to actually achieve? Because it's not something that there is any real need for.