Using mobile data while connected to WiFi. I've just been asked by the wife why her iPhone 5s (iOS 9.1) is not connecting to the wifi. I've just checked the router and it shows that her phone is connected and given an IP address, however it still shows 3G. 

Things I've tried. 


Turn her phone into airplane mode and not. 
Reboot the router. 
Repeat step 1. 
Restart the phone. 


I know the phone is connecting to wifi because when I disconnect the phone it disappears from the connected devices list and when I connect again it shows up again. It also shows up in the phones settings menu with a blue tick next to our wifi connection.  

Any Ideas?

Mark
The latest ios9 has a wifi assist feature.  If the phone thinks you're on a lousy wifi connection it will failover to cellular data.   I've turned mine off.

Settings > Cellular > Wi-Fi Assist

See https://support.apple.com/en-us/HT205296