This Forum Beta is ONLY for registered owners of D-Link products in the USA for which we have created boards at this time.
What state are you in? Native IPv6 is just being rolled out and only available in 4 Cox areas. I know because I also have Cox and waiting desperately for it to come to my state. Is it possible the DHCPv6 server is not giving you a complete IPv6 block. A good thread to check out is the one on DSLR.Also, I think it could be a issue with the firewall routing the complex /64 to the LAN.
Also a side note, this model router is of older generation code. The code may to be fully developed as newer generation routers are. I know there was a few changes in FW for other model routers regarding IPv6 over the years and I don't think the 615 received any of these updates.
I'm in the San Diego market.. I posted in that thread at DSLReports and you suggested that I try here.