Is it a subnet level address or only a single interface level address. Many ISPs, in the US at least, don’t supply a block of addresses, just one. If you’re only getting a single IP address or it’s using 6rd (like my ISP) or similar really old, transitional technologies, then you won’t be able to assign a unique GUA to your LAN (separate from the WAN address) and so you’ll need additional routing configuration like NAT to properly route to locally defined IPv6 addresses.

Personally, I gave up on trying to get things to work with 6rd since NAT is way easier on IPv4 since it was never intended to be necessary on IPv6. But ISPs are just cheap, lazy, or actively trying to make self-hosting on residential plans more difficult in order to drive people to business plans and prevent businesses from using cheaper residential plans by keeping IP addresses dynamic and thus complicating DNS.

If you are getting a subnet of addresses, then ensure that configuration on the LAN side is set up properly https://docs.opnsense.org/manual/ipv6.html

shot in the dark, because I’ve made that mistake before, if you got dyndns setup, it needs to be from the server not the router for ipv6 to work.

Are you using a VPN? A lot of the VPN vendors disallow IPV6.