Do you know the transceiver and ethernet cable to be good?
Have you tried assigning a static IP on the client?
What make/model is the router? Can you force the port connected to the client to a lower speed (1G) for testing?
what is the output of commands like lspci, ethtool and ifconfig, and what is in the network log?
You say “No network devices are seen” in Ubuntu. Was that also true of Proxmox? Or did Proxmox show the NICs, but just not get an IP? Just to be clear, what was the “best case” so far; that the NICs are not detected at all, or that the NICs are detected but no IP is being assigned?
As @DigitalGarden suggested it is likely not a software issue with the OS itself and is likely due to a compatibility issue with the transceivers and onboard NIC.
When you connect the trraisiever to the system check the “dmesg -T” logs to see if they say anything about an unsupported adapter