Looks like the issue was a combination of WPA2 snd the webportal. I have removed the web portal and it works.

Model : Hardware Version : Firmware Version : ISP : I have installed a CAP1750 as well as a CAP300 Outdoor. The 5 Ghz SSID works fine with all devices supporting 5 GHz band. for the 2.4 GHz SSID only...