It has recently come to light that your cast device may be killing your Wi-Fi. Google support said they will be rolling out an update to fix the issue on Thursday the 18th.
Due to a bug within the Cast software, some Android devices are sending a large amount of network traffic which is clogging up Wi-Fi networks.
People with an Android phone and a Chromecast built-in device (such as a Chromecast or Google Home device) on the same Wi-Fi network may experience this issue.
Source: Google Support