Question WiFi issue with AVM Fritz!Box 7490


has anyone encountered any issues with the touchpad and the fritzbox 7490 when using the 5ghz wifi?
Wifi works just fine with the predecessor of the fritzbox, the 7390, but I can't get a connection working with the 7490. WiFi-Connection is there, I can obtain an IP-address but I can't ping the gateway from the touchpad or the touchpad from any other network device.
2.4ghz works fine with both routers.

Any Ideas on how to troubleshoot this?

I am currently running Android 4.3.1 (CM 10.2-20131203-UNOFFICIAL-tenderloin). But I tried webos as well, same problem: I can get an IP, but no internet access.

Any ideas?