Sophos XG on KVM on Ubuntu - Shield Host - How ?

Hi

I have opened a question on ubuntu forum:

https://askubuntu.com/questions/1102169/ubunu-server-kvm-sophos-firewall

I would like to configure my host (ubuntu server) to use the LAN of the firewall instead of directly accessing the phyical nic. all phisical nics should be accessed only by the sophos vm and all guest and the host himself would need to use the firewalls LAN.

Is there somewhere a tutorial? I followed the installation guide (Sophos XG on KVM), so far all OK. But the steps after are missing.