I'm sorry about the issue. Was it working previously? Or is this a new chromecast or new router, etc.?
What's the full error or message when connecting the Chromecast to your Wi-Fi/wireless network name?
Can I connect remotely to your computer, so I can adjust the router's setting in order to make it work with the Chromecast?
Do you have an extra Ethernet/network cable that you can connect temporarily from the computer to the router, if it's not connected yet?
Please reply at the box below. Thank you.