6.0 marshmallow - 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

Answer

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.