I was waiting to see if someone had a 'real fix' for you.
I'm offering a Work Around that I use for a different reason.
Install =>
* MacroDroid - Device Automation - Android Apps on Google Play
Now create a macro that will toggle with turning OFF the WiFi.
Touch the + sign to create new macro
Trigger: WiFi State Change
Actions: Mobile Data On/Off
Now touch the Right Arrow in the top right corner of the display.
- ignore the Constraints menu
touch the Right Arrow in the top right corner of the display again.
- Now touch 'enter macro name' to bring up the keyboard.
- Give it a meaningful name like: Data ON, WiFi off
Click OK
You should now see the "List of Macros" has a number in a circle indicating the number of macros created. In your case, it should be '1'
Now, back out of MacroDroid and toggle off the WiFi, you should see the 4g indicator come on again.
The neat thing about this is, if the WiFi signal disappears, the Data should automatically turn on again.
Please come back and comment on your results.