Hi Marcus,
Assuming that you have done the internal swap over to ethernet interface inside the Ping360 - as per here:
Noting precaution of “If the Ping360 is changed over to Ethernet communications, DO NOT plug it into a computer via the USB connection, it will damage the transformer on the Ethernet configuration.”
My first port of call would be to connect the ping360 directly to an ethernet router to which the topside computer is connected. By default the ping360 is in dhcp client mode, so will grab an IP address from the router and you can then at least confirm the function of the ping360. If you can then establish a connection, best to chang ping360 to the default choice of “192.168.2.4”.
In regards to bandwidth, can you confirm bandwidth without the ethernet switch in the loop i.e. raspberry pi connected by ethernet directly to ROV Fathom-x board. This will confirm if the switch is causing any issues.
You can also try a different pair in the tether to rule out an issue with that particular pair of connectors in the tether - need to swap both ends