In the meantime I have connected LinkSys RE7000 to my Fritz!Box without any problems. Simply press the WLAN button (= WPS button) for longer than 6 seconds (until the WLAN LED flashes), then the connection is automatically established. The range is clearly improved.
I have a FritzBox (7390) for telephone and WLan connection from Sunrise, so I am in a similar situation. Unfortunately, the WPS button is missing on it, the automatic configuration doesn't work anyway. So I will try a browser-based setup as soon as I have time. There are German instructions for this at
http://www.linksys.com/ch... (Manual setup of the Linksys RE7000)