I know nothing about TrueNAS so I can not be of much help but…
The app installation determined the 172.16.1.2 address and UrBackup then broadcasts on the address it has been given.
UrBackup seems to be running on a VM under some form of virtualisation, KVM, docker or whatever TrueNAS uses. I expect eth0 is a virtual interface to an internal bridge network that connects to the physical network via NAT. This method of connection blocks much of the network traffic used by UrBackup and requires ports to be opened to work. I think this is why broadcasts are blocked as they are local to the internal bridge network.
The network setup is a function of the TrueNAS VM settings rather than a function of UrBackup. I had a quick look at the TrueNAS documentation for UrBackup and saw nothing useful. They do have information on networking in general.
This is all determined by the networking setup configured during installation. I did a quick forum search and found Install UrBackup Server on Truenas Scale ElectricEel-24.10.0.2. Following this would create the kind of setup I suspect you have.
Another post, Install UrBackup Server on Truenas Scale, seems closer to what you expect.
The current setup treats your LAN as a WAN and you would use an internet setup for UrBackup. This should work provided you have opened the ports needed by UrBackup as given by the first post.
The second posts gives UrBackup a direct connection to your LAN and allow the server to detect local clients.
To get something better than my guesswork you would need to post more of your app installation details.