I just had a Cox contractor come out to check my Internet because I was having to reset my modem a lot lately. He told me about something called powerline from Netgear that is basically a wall adapte...
I don't know why a contractor would recommend an Ethernet solution for a modem problem. It's 2 different networks.
If you're having signal issues at your modem, the problem is somewhere between the headend at Cox and your modem. This problem could be with the coaxial cable itself...the primary wall plate in your living room...the coaxial cable from the wall plate to the NID (plastic box) bolted onto the outside of your house... and anywhere onward towards the headend..
Powerline is only a solution for your network, such as the connection from your devices to your router.