Presumably you are running this as a Jail?
What is your main network address for the freenas?
I run duplicati on FreeNAS and it works well. I seem to remember when I set it up I used NAT but then changed the jail properties so that the Jail used a different IP (static) on the main network, not the FreeNAS IP and not some other network that I don’t think would ever work due to no routing.