Throughput testing with iperf

I've got iperf loaded on a number of nodes in our area, to do throughput testing with.
While I can do it on an RF link, it seems like port 5001 is blocked if I want to do it across a DtD link:
What I have is   MESH NODE <---- RF link----> MESH NODE <--cable--> AirOS NODE <---RF link--> AirOS NODE <--cable -> MESH node.
I'd like to do end to end throughput testing, but it's not working.   nmap says port 5001 isn't showing up on the distant node, even though iperf is running there.
What can I do to unblock this?

Thanks.