NAT network configuration

Hi guys, hope you’re well!
I have two LANs, first is and the second is
The UrBackups server is
One of the client is a server on
Both networks connects with each other via linux-based PC which has two interfaces (let’s say and and this PC is not a gateway for network), so I have a kind of a bridge between these two networks.
The problem is I cant discover client using “non-Internet” mode. Of course I could use “Internet mode” but it’s VERY slow (I have to make initial backup ~200GB and the speed is about 1 Mbyte per minute). So I do want to estabilish LAN connection. Could you please advice me how to do it? I have iptables installed on “bridge”, but as I found in manual, there are “UDP broadcast messages” and something else a little bit weird for me.