The sender and receiver use a particular local/network relationship; leaving that network can affect control or the playback route, depending on the app.
Why this happens
A phone that switches from home Wi-Fi to mobile data is no longer in the same connection situation as when it started casting. Some sessions let the receiver continue fetching media, while other workflows depend more directly on the sender. Screen mirroring is different again. The word casting is often used for all of these, but they do not have identical requirements. First identify which route the application actually uses and whether playback stops or only the phone’s controls disappear.
Google’s casting instructions specify the sender and Cast device being on the same Wi-Fi network for the normal supported workflow. Follow the app’s own instructions for any behavior beyond that. Compare one session without allowing the phone to change networks and keep the receiver powered and connected. If the issue appears only when the phone sleeps, note that separately from a Wi-Fi handover. A native player on the television can be a useful alternative when you need viewing that does not rely on the phone’s current connection or screen state.
A useful way to work through it
Identify native casting, mirroring or another sender-dependent route.
Compare a session while the phone remains on the same Wi-Fi network.
Record whether the video stops or only the sender’s control connection is lost.