I have a similar model, I just push the mode button until I have sound that is from my TV. Here's a link to a manual that might help? https://www.google.com/url?sa=t&rct=...Aav9IaM1GtDXBA
The key here, is the Cornerstone ZX50 connected to the TV? If it has been recently replaced, good chance it's not.
As far as WIFI. Not much help there. I know that WIFI is only good for about 300ft from the transmitter, so you might want to ask where the repeater or transmitter is. Or ask if they could add another to boost signal?