Debian – Dual stack linux router, can’t forward IPv6 prefix

I have a small offic here with a few workstations and I would like to implement a small server for stuff like a proxy server, samba etc.
This is the topology of said network:

+-----+   dynamic    +--------+        +-------------------+                 +--+workstation1
|     |  /56 prefix  |        |        |   Debian server   |        +--------+
| ISP +--------------+ Router +-+eth0+-+                   +-+eth1+-+ switch +--+workstation2
|     |   dynamic    |        |        | (squid,dhcp,etc.) |        +--------+
+-----+ IPv4 address +--------+        +-------------------+                 +--+workstationX

I would like to have the server assign global (maybe local as well) IPv6 addresses and private IPv4 addresses to all devices connected to eth1 via the switch. I have been able to get NAT masquerading working with dnsmasq and iptables but I can't get IPv6 traffic or even distributing IPv6 addresses to work. The router has three different settings in the IPv6 DHCP server tab:

• assign DNS server only
• assign DNS server and IPv6 prefix to downstream routers (what I assume should be enabled)
• assign DNS server, IPv6 prefix to downstream routers and IPv6 address

I tried following this guide. It describes how to set up a DHCPv6 server with dnsmasq. I have a working eth0 which gets a global, unique local and a link local address. But my eth1 interface never gets a global IPv6 address, only a link-local one. The devices behind the server all have a working IPv4 address but I can't set the IP to a static one because the prefix is dynamic and changes every night. Currently I'm stuck at this point.

Here are my configuration files.

Thank you for your help.