Enable IPv6 on your Wan is the first step in this process:

Goto your 'Interfaces' -> 'WAN'

Select for your IPv6 configuration ' DHCP6'

in the 'DHCP6 Client Configuration' select '56' for the 'DHCPv6 Prefix Delegation size'

Select 'Do not allow PD/Address release'. This should prevent that the Prefix is changed after reboot.

You should be able to receive a IPv6 address on the WAN connection.

Check your 'Status' -> 'Interfaces'

you should see an IPv6 and IPv6 link local address

LAN

Now for the your LAN interface configure 'Track Interface'

Configure the Router Advertisment:

(I'm running 'Assisted' because I need some static ip's in the IPv6 range) -> most people should select "Stateless DHCP" here.

Next we need to add a Firewall rule for it to start working:

(on your firewall rules on the WAN connection add:

You should see the IPv6 activated on your LAN and devices connected to your LAN

Optional ( But Recommend for IPv6 ):

To get a 20/20 score on https://ipv6-test.com add the following Firewall rule on the WAN