I don't think it is a hardware problem, I think it is a network issue. I have the miners hooked up to the router, and i cant get on the panel from my computer.
If your computer is on the same subnet as the miners, they should work just fine. Also some miners, you might of received them set on static, on 1.99. If you're not in the 1.x subnet you wont be able to connect to them.
Its also possible the previous owner messed them up so you could try a reset, if you can't find them in the 1.x subnet.