Replace PPPoE bridge

[dennes]

Hi,
The issue.. (long read)
My ISP uses PPPoE to connect to their fiber network.
In the current setup we have:
WAN fiber switch –-----> Siemens Gigaset PPPoE router ------> Sophos UTM firewall

The Gigaset only does PPPoE bridging and uses the first public address in my /29 subnet. The other addresses can be used statically on the Sophos.
Now the Gigaset supposedly only supports up to 20Mbit and we upgraded the fiber to 50. We want to replace the Gigaset but the ISP only sells a very expensive Cisco to take over the task.
No problem you'd think, i'll just let the Sophos do PPPoE. But the problem is that with PPPoE the Sophos automatically gets the first addres in the subnet (previously taken by the Gigaset and the Sophos' gateway address) and this obviously cannot be changed. I can use the other addresses too in my Sophos, bu the interface is set to the first address, which is a different address than before and thus my VPN onnections to other sites stop working (they can only be set to the if address).
Changing the VPN sites is out of the question (theres a LOT of them).
So i basically want to replace the Gigaset with an SG1000 in WAN bridge mode.
So the Pfsense should do the PPPoE and take up the first address in my subnet. Allowing the Sophos to use the other addresses in the subnet and the PFsense as the gateway.
Can this be achieved? Is this just a question of setting the PFsense to bridge WAN-LAN?

Thanks,
Dennes