1. I know the device itself (hardware) should be capable of it. To bad those functions are not implemented in the software. Will they be implemented in the future or is the only option to custom flash it?
No.I now configured it as a separate access point but with the same SSID/AES encryption as my primary AP. That gives my devices "roaming" access.
2. But it brings me at another problem. I noticed that now the DHCP server on the DLINK is disabled the MAC address filtering does not work anymore. e.g. if I turn ON the Mac Address Filter to only allow specifiek devices I cannot get a DHCP lease from my primary AP. If I disable the Mac Address Filter it works just fine.
If you have the 855 running in AP mode then it's preferred to turn off ALL extra features of the 855, i.e. filtering, SPI, WISH and everything and run it as lean and mean as you can. The host router that the 855 is connected to will handle and should handle all of the DHCP, filtering and rule services.
Turning a router into an AP.*bug* ?
No.