Why is my phone using 4G instead of WiFi when it's on?

[AC Question]

I have an S7 and so does my wife.

She's been sucking up data because it hasn't been using WiFi. I've finally started looking into it, and this is what I've found:

Both Devices can use both networks just fine when only one is on.
i.e. WiFi works when ONLY WiFi is on and Data works just fine when ONLY Data is on.

However, on MY phone, when I have both, it uses my WiFi network. On my wife's phone, I'm unable to instruct it to use WiFi first and THEN try to use Data if the WiFi connection is shotty.

I'm on Rogers up in Canada. Can anyone help?

 

[chanchan05]

Do you have download boost on? AFAIK that makes the phone switch to what it perceives as faster connection.

 

[capvincenzo]

Good to know, but we both do not have our Download Booster on.