New Macs will not recognize ethernet

OK, so I'm at my wits end here. Last month I purchased three brand new macs to upgrade the computers in my house - a mac mini for the kids, a mac mini (with upgraded 10gb ethernet card) for my office, and an iMac for the wife. My house is wired for ethernet.

Here's the problem - NONE of the three new macs will recognize when I plug in an ethernet cable. I can only connect to the internet over wifi (which works, most of the time, but for speed and security reasons, I need ethernet to work).

I don't think it's a problem with the modem, or the router, or the ethernet wired through the house, or the cable I'm using - I know this because I have an ancient 2012 model Macbook with an ethernet port, and I can plug that in wherever I want and it sees the ethernet right away. I'm instantly online and good to go. BUT - when I take the same cable and connect it to ANY of the new 3 new macs, NONE of them recognize that I've plugged a cable in.

I've tried booting in Safe Mode - doesn't matter. Still don't see the ethernet cable. I've explored the "Incompatible Kernel Extension Configuration" bug that was reported back in 2016, but that's not it (I don't have version 3.28.1). I've confirmed that all three macs do, in fact, have ethernet cards installed (via "About this Mac" --> "System Report"). I've tried removing the Ethernet connection from the list of options on the Network window of System Preferences, with no change. I'm literally so frustrated here, and kind of think it has to be either something with my router/modem that just doesn't talk to newer macs, or something about Mojave - because it's just too weird that all three macs, all brand new, and all with different hardware configurations, would have the same problem.

Please help!