In principle, you can connect the repeater to any WLAN router. However, if it is connected to a Fritzbox router, extended functionalities are available. For example, the repeater can take over certain settings of the router if desired, or you can change settings on the router and the repeater will take them over automatically. So if the sunrise internet box is the model Fritzbox 7390 or 7490, this extended functionality will be available. Otherwise, the repeater works like any other repeater....
I agree with the previous speaker. The FRITZ!WLAN Repeater 1750E basically works with any router (WPS). I use it with the Swisscom box. No problems, great reception and fast!