Switching between wifi and mobile data connections

Jammy@apple wrote:

Thanks for your reply.

Yes,router will be working fine but connection to internet goes away. In such cases, have to manually turnoff wifi on my iPhone.

That is expected behavior. Your phone will only switch to cellular if it loses the WiFi connection. Losing the internet connection won't do it.

When you say Wi-Fi router is not working do you mean the entire network goes away or just the internet stops working?

A lot of times with Wi-Fi issues the network does not go away, only the connection to the internet breaks. An example would be the pipe stay connected but there is no water flowing through. If this happens there is still a Wi-Fi network and your phone will try to stay connected even though there is no internet.

You should try to get your router/internet service fixed but in the meantime you will have to keep disabling Wi-Fi manually.

If you tap the Wi-Fi button in the Control Center it will disconnect from the current network, but still leave Wi-Fi on so it can connect to a different network and can still work with Location Services (see: Use Bluetooth and Wi-Fi in Control Center with iOS 11 - Apple Support)

Something to try: Go to Settings/Cellular, scroll to the bottom, and turn on Wi-Fi Assist. This may force a switch to Cellular when your Wi-Fi performance is lower than Cellular data. It may help in your situation.