What about setting static addresses for vmbr1 on proxmox and for red interface on Nethserver VM?
Just an idea…
Changing your config from proxmox being just a server with bridge to Nethserver VM, being just a server to proxmox is a router now and Nethserver VM too, is not so easy.
Sorry, I don’t have a Proxmox test system… 
Here some networking/routing tips for Proxmox:
https://pve.proxmox.com/wiki/Network_Model
More to read…
Wiki:
https://wiki.nethserver.org/doku.php?id=userguide:install_nethserver_on_proxmox
Installing IPCop(another firewall) on proxmox:
https://joealdeguer.com/installing-ipcop-as-a-virtual-machine-on-proxmox-ve/
Another thread with no internet on proxmox: