Hmmm, it's a different format that I'm used to, but IPv6-PD: 2a01:90a0:1ff:fb00::/56 suggests to me that that @nio has a /56 delegated. I think that the address on WAN6 is that of the ISP's router's subnet.
Can you ping your gateway from your OpenWrt router?
ping6 fe80::1%eth0.2
(IPv6 link-local addresses don't need to be unique, the %eth0.2 says that it's the fe80::1 seen on that link.)