Hmm… unfortunately there is nothing else I can think of. My WAN is setup as DHCP in the MV1000 and I have mine connected to my main Asus router (not my ISP’s modem). I have IPv6 disabled in my main router. The MV1000 gets both IPv4/6 addresses from my VPN provider and all clients connected to it can also use the VPN’s IPv4/6 addresses.

Have you tried to connect to your WG server through your phone or PC and see if you get an IPv6 address?

I bought the MV1000 because it is one of the few consumer-grade routers that support IPv6 over VPN. IPv6 addresses are less likely to be blocked by services online (for now).