Does your TO get a single IPv6 Adress or a IPv6-Block? Is IPv6 Prefix-Delegation enabled on your Fritzbox (you can google it, it may give you some explanation how to configure a „chain of routers“ with IPv6.
Edit: Tutorial for Fritzbox (German): https://avm.de/service/fritzbox/fritzbox-3270/wissensdatenbank/publication/show/1239_IPv6-Subnetz-in-FRITZ-Box-einrichten/
TO should work with prefix delegation in standard configuration for lan-interface (at least in my setup behind another provider-router with ipv6 prefix delegation enabled)