Altibox do support IPv6 to the home.

In fact - they support it on most modems - 5 of the 6 here are listed.

In my case - the FMG modem is listed - but in bridge mode we need to configure the network correctly.

It turns out that they are providing a standard DHCPv6 connection - with a prefix delegation size of /56.

So - there are two parts to this - one is to configure the WAN interface - the other the network.

Note!

Both originally and also recently after I restored my controller from backup (disk switch) - I did not get IPv6 straight away.

In both cases - after I power cycled the USG 4 Pro router - it all started to work.

WAN settings

So - in the network application on the unifi controller - under settings > internet > primary

Network settings

Now - for each network that is to get IPv6 (for this example just the default network) - settings > networks > default

My understanding of the prefix ID is it divides the supplied net into /64 blocks. So - ID 0 is ...::0/64, ID 1 is ...::1/64 etc. I just took 1 - any /64 is more than I need - but - if wanted - you could use different IDs on different local networks.

Under IPv6

Public DNS servers