I added all my local nodes as gateways via the local network IP to all other nodes. Though, I have only 3-4 running nodes.
I am not sure, but when at least one node has synced, your other nodes can get the consensus over the lan from that one node.
1.3.0 nodes connect well within the LAN, the problem is when NAT interferes.
so just gateway connect <local IP from the "main" node>:<RPC port on that>