Starting with P5 version 5.5, client to client synchronize plans are supported.
When a sync job is defined to transfer data from one P5 client to another, i.e. not to/from the P5 server (localhost) itself, P5 attempts to setup a direct connection from one P5 client to the other. To do so, the P5 server sends to the source P5 client the IP address, TCP port and credentials of the target P5 client as configured in the client settings on the P5 server.
This implies that the clients must be able to see each other with those values.
Specifically, it implies the source P5 client must be able to connect to the target P5 client using the TCP IP address that is set for the target client in the client configuration on the P5 server. In case the source client and the P5 server are not in the same network, that may fail, as the target P5 client may be visible at different IP addresses.
One way to resolve that would be to specify a DNS name instead of an TCP/IP address in the target P5 client settings on the P5 server. The P5 server and the source P5 client would then both have be able to resolve that given DNS name to the correct TCP/IP address, but the resolution may differ on both machines.