Using Raspberry Pi as a NAT

Perhaps not a question related directly to Ignition but…
I have a Raspberry Pi and wanted to use it to NAT to a different network (what the 9300-NAT does but at 1/10th the cost.
Using the PREROUTING command I can ping the PLC, RSLinx see’s it and I can go online and edit through the Pi across two networks.
(Using POSTROUTING as well seems to have no additional affect)
I even tried Kepware and that has no problem seeing it and can read and write to the PLC through the NAT.

The problem is Ignition will not connect. Wireshark shows a pattern of RST (resets I believe??) highlighted in red. The info for these are all ‘Ethernet-IP-2’ etc etc, protocol TCP.
Complete NooB at Linux and networking for that matter.
Tried a couple of Linux forums with no luck, hoping someone may shed some light on it :scratch: