Had the same problem. But I definitely know how to fix it!
1. Swipe down to Control Panel then hold on Wi-Fi
2. Next hit Details then go to the top right corner of your screen, you want to hit the 3 dots then select "Intellegent Wi-Fi"
3. Switch off "Turn On/Off Wi-fi Automatically"
Should of Resolved the problem. This only happens when signal for WiFi is low it will turn off to use mobile data.
😊 🙏

