@Hibiscus234
You can follow this guide for using the WDS feature to extend your wireless network coverage.
Remember that only when the WDS status is 'RUN', the connection is built successfully.
When setting this up, do it while the extended device is not far away from the main router.
Once you have successful connection, place the extended device where you've intended to.
If connection can't be established, then maybe the distance is too long for the wireless signal.
If you've already done all these and still have no internet on your Chromecast, then try changing your ISP provided DNS with 8.8.8.8 / 8.8.4.4.
If that still doesn't help you can follow this guide.