When you connect to the WiFi did you turn off mobile data on your phone?
Android Phones automatically switch to Wifi when wifi is enabled. Except if you on "Smart Network Switch" or "Download Booster". To prevent it, just turn off your mobile Data as suggested.
Smart Network Switch - The device will automatically check which is more stable and fast beetween WiFi and Data
Download Booster - The device will combine the speed of the two network connection (Wifi and Mobile Data) to provide seamless connectivity.