Hi,
first the link: ftp://ftp.dlink.com/Gateway/dir855/Beta/Firmware/dir855_fw_121b07NA_beta.zip
It works

I made better experience with auto channel scan in the 2.4 GHz frequencies but other users reported that only manual channel selection worked for them. So you have to try out what works best for you.
Best channels to use depend on what is used in your neighbourhood. Install a program like inSSIDer, Netstumbler or Vistumbler and you have an overview.
As each of the 11 (13 in Europe) channels has only 5MHz bandwidth but WLAN sends with 20 (40 with channel bonding) MHz it will occupy for example the channels 4,5,
6,7,8 if you use channel 6.
So the three non overlapping channels are 1, 6 , 11
Best is to leave 5 channels between your neighbours wireless and yours - if possible.
Else use the channels with the weakest signals found by the programs.
For 5GHz you need to take less care as all channels are non overlapping. If your router does not use a free channel on its own, just select one manually.
But there is less traffic.