Hi,Check this. Settings>Connections>Wi-fi>Advanced>Switch to mobile data*>OFF. *Or anything similar to that.
If you don't bother, reset the network settings. Settings>General management>Reset network settings. (it will delete all saved networks, passwords, APNs and paired Bluetooth devices.)
Also check after rebooting in to 'safe mode'. Here you can check whether the problem is with any downloaded apps.
If the problem is with only a single wifi network, then it will be an external issue. Restart your router. Also check for firmware updates from router manufacturers website.
