Finally I solved my wi-fi problem that I was having upstairs. In the bedroom and bathroom I have a weak signal, and sometimes in the middle of a page reload or a game load, it would switch to my providers network (3G). Upstairs in the computerroom or on the roof-terrace I have no wi-fi signal whatsoever.
I thought, why not try the old Siemens SX762 ADSL router, and just skip the ADSL part? Turns out it can be used without ADSL, and LAN-port4 can be used to uplink. With that port connected to my switch upstairs I now have wi-fi there. It’s only 54g, but good enough for the phone.