G'Day.
I initially had similar problems with the 865L until I tried this...
Hard wired the 865 directly to my PC, and connected using the usual 192.168.0.1 address.
Then in the Network setup screen...
Router section: switched ON the DNS relay (so it gets the IP address from my cable modem once connected)
I manually set the 865 IP address to 192.168.0.101 (outside the usual range of addresses)
Next, I DISABLED the DHCP server (eliminating the double NAT issue)
Disconnected from the PC, and connected it to the Cable Modem.
Everything is OK now (except I can't register with mydlink)
Regards, Greg.