Routing when only having IPV6 in place

Hi together,

for connecting to a remote server (remote desktop) with only IPV6 I enabled IPV6 on a Sophos UTM 9.5.

1) I configured my first free address from my given /64 address range with is :2 with a /125 mask at the end.

:1 is the ISP interface.

After this I can reach my external IP using ping from all PCs I tried (and having IPV6 enabled).

2) So I configured :9 /125 for the internal interface and :10 /125 for my internal server.

But I am unable to reach the Server from Sophos and vice versa.

I understand that there is automatic routing done by my UTM between the interfaces when I don't specify any NAT maskerading (right?).

I defined some needed firewall rules but nothing better.

Funny: Trying ping from my server to the internal sophos interface : packets are lost and also no entry in sophos fw log

But: Trying ping from my server to microsoft ip address: pakets are lost but this try is logged in sophos fw log

Also trying ping or RDP (3389) to the internal address: No entry in sophos log

ISP Gateway <-> Sophos external <-> Sophos internal <-> internal Server

...z:y:x:1/125       ...z:y:x:2/125             ...z:y:x:9/125          ...z:y:x:10/125

any idea?

My first (actual unsuccessfull) IPV6 tryings...

Thanks for help