Well, another matter of concern might be the presence of an IPv6 firewall within your router. According to the manual (last available version from 2013) it shouldn't have one (which is bad for your LAN clients, because with IPv6 they can be directly accessed from the Internet, so they should have host based firewalls). But maybe with the latest firmware, there is a builtin IPv6 firewall. In this case you have to activate it and configure a single rule that allows all IPv6 traffic LAN --> WAN (that's the strange thing with D-Link firewalls not providing a foolproof working default setting).