You might be able to get another cable modem working from the same coax; I've done it with Cox. But one thing is that I own 8 static IPs, I'm not sure what would happen with the standard DHCP single address that is typical for residences.
I would think that the cable company would want to charge you for the extra IP address, who is the line provisioned from?
With the powerline failure, is it possible that you have the two adapters on two different phases of the circuit? If so, you can see if you can find a outlet with the same phase, or you can buy a phase coupler. This one is nice as you just connect it to the 220 service.
Maybe you can run something on the outside, getting out where the cable coax exits and down to the basement?
If the Wifi problem is because of old, thick plaster walls, you might have better chances running the radios near the windows, and even use a small external directional antenna. Glass is easier to get thru than old walls. Try turning off MIMO, and just use G, sometimes it's better when you just want to go point to point.
If you have coax, you can use a MoCA device which sends the network over the coax, after the modem.
Many isp are using this to place additional cable boxes on the IP network over coax.
There are consumer devices to extend a network through the home wiring.
IPVM | IPVMU Certified | 05/18/16 02:05pm
Here is what I am using at home to provide Netflix, Pandora, YouTube, MLB.tv, etc / internet access at my cable boxes. The device is provided by my provider.