IPv6 support is configured by network interface in Network > LAN Interfaces:
The following configurations are available:
Equivalent to former versions of IGEL Linux: NetworkManager ignores the device, but the kernel performs some basic configuration. In particular, it assigns an IPv6 link local address to the device.
IPv6 is disabled completely.
The device tries to perform an IPv6 stateless or stateful autoconfiguration based on router advertisements. Depending on the router advertisements, this involves DHCPv6 (see RFC 4861).
This option is supported by NetworkManager. It can be used when a DHCPv6 server is available but no router advertisements. Routing has to be configured by other means. In most cases Automatic will be preferable.