How to connect to WIFI but use phone mobile internet? My WIFI internet is very slow. 
I would like to use my mobile internet of my phone, but connect to my wifi because it has Google Cast in it. 

Phone: Oneplus 2
OS: OO 3.1
There should be an option called Smart network switch which switches to mobile data connection whenever the WiFi connectivity is limited or unstable.

The screenshot below is from Samsung Galaxy.