Chromecast needs a WiFi connection to stream content to the connected TV. This means you need an internet connection and a WiFi router. For those who do not have any of these, a mobile hotspot was the workaround to use Chromecast.
A recent update to Google Play Services, Play Services version 11.0.55, makes it impossible to use mobile hotspots to cast. When asked in the Google’s Chromecast forum, the reply was that it was never officially supported:
Mobile hotspots are not officially supported for Chromecast. Chromecast needs access to a secure, wireless network with internet access to work as intended.
Google’s recommendations for a Chromecast set up includes a “secure wireless network”:
We recommend that you use a secure network with your Chromecast device.
While it’s good security practice in general, using a secure network will also help prevent others from joining your Wi-Fi network without your knowledge and casting content to your TV. Be extra careful whenever you go online using a network you don’t know. Learn more about setting up your home router and Wi-Fi network securely here.
This almost means that this feature is never coming back. Unless someone finds a workaround to make this workaround to work!