IPv6 prefix delegation to LAN

[Bentrax]

Hi community,
I tried everything I found the last few days to get IPv6 running on my LAN.
I have a double router configuration, a Speedlink 5501 in front and then a pfSense connected.
The pfSense WAN side is on the LAN side of the Speedlink.
The Speedlink has RA enabled and runs radvd with following config:

interface eth0
{
 IgnoreIfMissing on; 
 AdvSendAdvert on; 
 MinRtrAdvInterval 5;
 MaxRtrAdvInterval 100;
 AdvLinkMTU 1492;
 prefix [PREFIX]:a300::/64 {
 DeprecatePrefix on;
 DecrementLifetimes on;
 AdvValidLifetime  14396;
 AdvPreferredLifetime  1796;
 };
 route [PREFIX]:a300::/56
 {
   AdvRoutePreference medium;
 };
 RDNSS  fe80::5ef4:abff:feff:ae8  {};
};

It gets its IPv6 address with DHCP6.
I want to delegate this prefix to the LAN on the pfSense.
I configured the WAN side of pfSense to DHCP6 and it gets an address, I configured the prefix to /64 as obviously the Speedlink only passes /64.
Then I enabled "Track interface" on the LAN IF set to "WAN" and "0" since there is only one subnet available.

But the LAN side does not get an address and radvd on the pfSense always has this error:

radvd[24790]: no auto-selected prefix on interface vr2, disabling advertisements

Where is the point I am missing?
If the WAN pfSense side does get an address and prefix why can't this be delegated too?

Any help/hint appreciated…

Regards,
Alexander

[razzfazz]

You can't use overlapping prefixes on the WAN and LAN side of your pfSense box ([PREFIX]:a300::/64 is a subset of [PREFIX]:a300::/56).