I'm on Unraid 6.12.13 and I'm having the same issue. I added a new PCIe network card with 2 NICs. They both show up in Unraid. I can configure them and they both work. The built-in NIC is eth0, and the network card is eth1 and eth2. Despite this the Network Rules tab does not show up.
I manually added the file /boot/config/network-rules.cfg with the correct configs. This forced the Network Rules tab to show and I can reassign NICs to different names. But of course this is not permanent and as soon as I reboot the file is deleted and the Network Rules tab is gone again.
I tried adding the /config/network-rules.cfg file directly to the flash drive but that also gets deleted when inserting the USB flash back into the server and booting up again.
I deleted /config/network.cfg directly from the flash drive to reset the network. That did rest the current configuration settings and I can see all 3 NICs but still no Network Rules tab.
I disabled the onboard NIC from the bios. Doing so automatically moved the network card first NIC to eth0 and second NIC to eth1. Even Though there are 2 NICs in the machine I still don't see the Network Rules tab.
Notes: Not sure if this is helpful but the Unraid USB flash was in a different server 2 years ago.
For now I'll keep the built in NIC disabled until the new version of Unraid (7) is released and I'll try again.
I welcome any tips. Thanks