How to setup a IPv6/64 subnet from a /56 from my ISP

Hi,

I'm not too familiar with IPv6 yet and I'm struggling with my setup.

I got assigned a subnet 2001:xxxx:yyyy:zz00::/56 from my ISP with 2001:xxxx:yyyy:zz00::1 as the gateway.
I assigned 2001:xxxx:yyyy:zz00::2/56 to my WAN interface and can ping that address from/to the firewall in both directions.

Now I want to use the subnet 2001:xxxx:yyyy:zz01::/64 on my internal network.

I assigned the 2001:xxxx:yyyy:zz01::1/64 to my internal interface and 2001:xxxx:yyyy:zz01::2/64 to a connected computer with 2001:xxxx:yyyy:zz01::1 as the gateway.

I can't even ping between the firewall and the computer and of course there is no internet connection.

What am I dong wrong and how can I fix this?

Best regards
Martin