Have you tried looking in settings? The setting responsible for automatic switching between wifi and data can be useful but annoying if the wifi is slower than your data. To turn this off:
* Long-press the WiFi tile in the quick settings section.
* Then, select Wi-Fi preferences.
* To disable the feature, deselect “Auto switch to mobile data.”
* Alternatively, enable “Ask before switching” to prevent Wi-Fi from disconnecting without your permission.
best of luck ^-^