Hello, my name is***** is a budget brand, meaning they use cheap parts to build them. Their digital electronics such as the HDMI board inside the TV is not built very well, and they do tend to go out from power outages. That means a cable most likely won't do any good. However you can try different HDMI ports, because sometimes the damage is limited to certain HDMI ports.
If you use a screw in coax cable, that uses a different circuit board, and it is full HD. That input is not related to the HDMI board.
Additionally you can use red/green/blue "component" cables to get 720p, which is still HD.
To prevent damage in the future, if there's a storm coming in, and you think the power may flicker, unplug your electronics whenever possible.
It doesn't sound like they are selling you anything you don't need, in fact, it sounds like they are handling the situation like they should. HDMI boards fry in Vizio TVs very often.
If you have the Roku model with only HDMI, click here. This will convert the signal to component, and you will be able to have Roku. Since component is not digital, it has no relationship to the HDMI board.