Searched an extender for the dual-band (2.4 and 5 GHz). First try as a lot of good reviews was a unit of AVM Fritz. Unfortunately could not be contacted for Speedport (neither WPS still on the PC). At the end I told the AVM support that it will probably not go, since both devices just do not "like".
So the search went on, and by chance I came across Netgear. The values on the paper were promising, so I've ordered. The delivery was faster than expected (new device on the market). Then: Unpack, plug it in, press WPS button on the Extender, WPS button on Speedport - DONE! Full reception down to the last corner of the apartment. So wonderfully uncomplicated I've liked. The appliance expands both networks (we use 2.4 and 5 GHz simultaneously) and uses the same wireless network name, but with the addition 2GEXT (2.4GHz) or 5GEXT (5GHz band) behind the name of your choosing of the local WLAN. So you immediately recognize in which band and the device on which you are logged on. Perfect!
Thank you for reading!