Look at route64 and setup a tunnel then. I’m using it after switching from Hurricane Electric’s IPv6 tunnel broker. Setup is nearly identical except you can use the Wireguard tunnel for route64 and there are less blocks and issues compared to HE’s offering.

My backup connection is cellular and my primary is ftth but only IPv4 support.

Set the LAN ipv6 to track interface and it should work. You will likely also need to set a static ipv6 address on your LAN that is within the address given. SLAAC should take care of the rest.