Hi Alan,
Have you tried entering the "Peripherals section" on the BIOS at start up?
There may be a section for Ethernet or LAN connections.
Normally, it may be that a setting is disabled.
Or alternatively, a OS system will need to be installed either via disk or USB. Then you should be able to install updated drivers for the LAN/Ethernet connectors through either Windows Update or the appropriate installation disk.
By any chance, did you have an initial problem getting to the BIOS screen when installation of components were complete?