For a long long time, I could not make the IPv6 in the DSM to work. I tried and did a lot of tests to resolve this issue but all of them failed.
I did feel frustrated especially I had been upgraded the DSM version from 5 to 6.1, then to 6.2 just to fix it but still failed.
Today I fixed it because I found this issue was related with my bridged router, and in fact a simple manual configuration caused this issue.
My home network involves two NETGEAR R8500. The primary router is in the living room and the secondary one is in garage, which connects to the primary using wireless bridge mode. And the IPv6 issue is really weird that all the physical machines connected to the second router work well and this issue only happens on the VMs, including VM on VMWare or VM on VirtualBox. All the VMs could get the right IPv6 address and default gateway and they could reach the gateway with the Link-local IPv6 Addresses, but they could not got further. It seemed their IPv6 requests were not forwarded by the primary router.
Today I created a new openSUSE VM on VirtualBox and bridged the VM NIC to laptop wired NIC, then I found it also could not access the outside IPv6 addresses, so I reduplicated the same IPv6 issue on a new VM. Then I tried to change the bridge network to wireless adapter so the VM could get the IP address from the primary router directly, then I did the ping test again and found it worked! So the issue was about the second router in the garage.
To access the second router, when I configured the wireless bridge mode, I assigned fixed IP address to the router and a IPv4 default gateway. I could not find the corresponding IPv6 configuration so I just let the second router get the IP address and gateway info automatically, then I changed the bridged network on the VM to original wired network and tested the IPv6 again, it still worked well. I did the same test on the DSM and it worked also!
So the issue was caused by the wireless bridge mode on the second router when I configured the IP address manually. The interface only provides a IPv4 address and gateway so I could not assign a IPv6 gateway.
I still could not explain why all the physical machines works well under the same router.