How to restrict the apps to use data in the background?

For Funtouch OS 9.0 and higher version, go to Settings>(Dual SIM cards and )Mobile network>Data monitor), then you can restrict the apps to use data in the background.

For the below Funtouch OS 9.0 version, go to Settings>(Dual SIM cards and )Mobile network, scroll down, and then you will find Data-saving mode.

Warmly Tips: It is not available in Funtouch OS 3.0 and lower version. But don`t be upset, there are some tips for you to save data.