If your Wi-Fi connection is unstable, the transfer may switch to cellular data.
Before initiating the transfer, it is recommended to enable 'Only when connected to Wi-Fi' option.
Enable Only when connected to Wi-Fi
1. Tap ☰ menu icon in the top left corner.
2. Tap ⚙ gear icon in the upper right corner.
3. Under File Transfer, enable Only when connected to Wi-Fi.