Help needed with IPv6

I would kindly ask for some help with a Mikrotik hAP ac which is configured with IPv4 default config and NAT for the internal devices (and I cannot reset this)
Now I have to setup the Mikrotik with IPv6 because 1 device from internal network has to connect to outside network over IPv6. I got a /64 prefix " and want to set up a fixed IP on the internal device, the router LAN interface and the router WAN interface.
The gateway for the WAN interface is a firewall which has the first address from this prefix.
I tried some configs but every time I can ping only from the WAN interface and not from the LAN or any clients.
RouterOS 7.14 is installed.
The clients say that they have no route to the destination.
Some basic firewalls rules are in place.
I think I do not need any pool or dhcp.

Can someone give me a hint what I’m missing there?
Thanks,
Martina

This description is a bit of a hodgepodge.

Please read up on IPv6 first to better understand your situation, specifically address distribution (SLAAC), Prefix Delegation (DHCPv6-PD) and classification of IPv6 addresses in general.