Ethernet on 3rd Gen Apple TV

I cannot get my 3rd gen Apple TV connect to my network via the Ethernet cable. This used to work fine until my ISP (AT&T) installed a new high speed router/modem. I've gone round-n-round with AT&T tech support and they are pretty much useless as they keep saying everything is working properly on the router. The Ethernet ports are indeed working properly as I have multple Macs connected via the Ethernet ports. It's just the Apple TV's that no longer work over the Ethernet ports. Over wired Ethernet, I achieve a strong 84megs per second download speed which is totally ample to handle the Apple TVs. I actually have 3 different Apple TV's scattered throughout the house and I have tried all three (2 3rd gen and 1 4th gen) over Ethernet and all no longer automatically recognize the network nor do they permit any manual input of the needed network info either. Only wifi is available. And wifi would be an option, however, it drops signal WAY too much to be useful, which is why I'm wanting to use Ethernet. I have also tried swapping out the Ethernet cables to rule out "bad" cables but that isn't it either. Can you guys throw me a bone here? This is really quite frustrating.