Trouble to get Dual-Stack running

Hi there!

I am having issues to get a Dual-Stack up and running with the Sophos UTM and Swisscom as ISP.

They provide me (in theory) with a /48 Prefix, but one has to request it without requesting a IPv6 IP.
I`ve created a thread in the german forum already, but after ~200 views and 17 days without answer I thought it might help to post this again on the english forum.
(https://community.sophos.com/products/unified-threat-management/f/german-forum/104417/ipv6-pd-via-pppoe-passthrough-swisscom)

Lets describe the topology: I am connected to my ISP using fibre, they provide a sync. Gbit to me. The fibre terminates at a "Centro Business Box 2.0".
This device is configured to do a PPPoE passthrough. Means: my UTM is connected to it via copper and the UTM is performing a PPPoE Dial-Up.

This is running nicely, like its supposed.

But I can't use IPv6 at all at the moment. From what I can tell (try and error) I would have to configure the UTM in a way its not possible on the UTM.
I found this blogpost from Tux0ne describing how he managed to get it working with pfsense: https://www.tuxone.ch/2016/10/pppoe-passthrough-mit-swisscom-my-kmu.html

I think the following points are essential:

• Request a IPv6 prefix/information through the IPv4 connectivity link --> didnt find a similar option, maybe its default?
• Only request an IPv6 prefix, do not request an IPv6 address --> didnt find a similar option
• DHCPv6 Prefix Delegation size /48 --> didnt find a similar option
• send options ia-na 0, ia-pd 0 --> didnt find a similar option
• Non-Temporary Address Allocation --> I think this can be done by setting a IPv6 Address on the "External WAN" NIC

I am wondering if someone does know how to implement this? I would love to use IPv6 with the UTM.

Thanks in advance!

Kind regards