The first step is to find out what type of IPv6 service you have. Using a default configuration where wan6 is only proto dhcpv6 with no options will usually discover this. You need to know if the ISP offers a prefix and what size it is.
Also make sure your modem is operating in "bridge" or "passthrough" mode instead of as a "gateway" The gateway mode does routing inside the modem and it may limit the LAN side to /128s as would be expected if it were only endpoints and not routers.