I don't know if this applies to everyone. I don't use much data, so I use my phone's hotspot for internet. For whatever reason a few months ago, Google released an update that wouldn't allow casting and hotspot from the same device.

Fortunately, the newest version of Google Play services allows this to happen again. The original issue can be found here . Here's the link to the newest apk for Google Play services , since the beta isn't on the Play Store yet.