I would use Wireshark on both PCs and watch Ethernet traffic to see broadcasted ARP/DHCP packets to observe IPs. This could help identify Modem B's WAN IP address as seen by Modem A.
You'll likely need to set up port forwarding on Modem B regardless to allow the SSH/SFTP connection through. Specifically, you'd forward TCP port 22 from Modem B's WAN interface to PC B's internal IP address.
However, the simplest modern solution would be to use Tailscale to connect the two networks and transfer the files. Tailscale uses WireGuard to create a secure mesh network that can traverse NAT boundaries without manual port forwarding.
Note that depending on the RF modem capabilities and Tailscale's NAT traversal success, connections might route through Tailscale's DERP relay servers (over the internet) rather than directly over your RF link. If keeping all traffic local is critical, you may need to verify this behavior.

Hello everyone,

I’m trying to transfer files from PC A to PC B using WinSCP (SCP/SFTP over SSH), but the only link between them is a pair of RF modems. Each PC is plugged into its own modem via Ethernet, and the modems talk to each other over RF. Both modems are in their default router/NAT + DHCP mode, so each PC lives on a separate private LAN:

[PC A: DHCP 192.168.1.x] ←Ethernet→ [Modem A: LAN 192.168.1.1, WAN via RF]
↑ RF cable (TX←→RX)
[Modem B: LAN 192.168.1.77, WAN via RF] ←Ethernet→ [PC B: DHCP 192.168.1.y]

I need WinSCP on PC A to reach SSH/SFTP on PC B, but:

1. Modem-to-modem NAT isolates the subnets.
   – PC A can’t directly see PC B’s IP or Modem B’s RF-side (WAN) address.
   
2. DHCP is required** for another application—so I’d prefer not to disable or override DHCP on either PC.
   
3. I’ve attempted:
   
   
   
   • DHCP reservation of fixed IPs on each modem’s LAN for PC A and PC B
     
   • ARP scanning (ping broadcast + `arp -a`), Angry IP Scanner and nmap to discover Modem B’s WAN IP
     
   • Enabling Telnet/SSH on Modem B (no CLI access available)
     
   • Adding a secondary IP alias and host routes on PC A to force 10.10.100.2 out the RF-link NIC
     
   • Bridging Ethernet 3 (DHCP LAN) and Ethernet 4 (RF-link) on PC A, but it gave me a completely new IP address for that.
     
   • Attempting transparent bridge mode on the RF modems (not supported)
     
   
   

In each case, PC A still cannot ping or SSH to PC B’s network. Modem B’s RF‐side address remains hidden behind Modem A’s NAT.

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What I’d like to know:

• Is there a recommended WinSCP (or alternative) setup that can traverse two cascaded NATed routers over an RF link without disabling DHCP or physically re-cabling both PCs onto one LAN?
  
• Can I leverage WinSCP’s tunneling, proxy, or FTP-over-SSH features to punch through Modem A’s NAT, or must I manually port-forward TCP 22 on Modem B and discover its RF‐side IP somehow?
  
• Any tips for reliably extracting Modem B’s RF IP when its GUI doesn’t show a WAN status page?
  

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Thanks in advance for any guidance or configuration examples. I’m happy to share more details about firmware versions or modem models if that helps.

—J