Mobile internet data usage automatically when wifi troubles

I am using LENOVO ZUK Z2PRO Z2121 with MokeeOS -71.2-0180816-nightly.
My network setting is below.
-Wifi : on
-Mobile internet : on

*I don’t change these setting.

I use wifi in my room.
I use mobile internet at out side.

I want to save mobile internet data.
Normally, mobile internet data is not used in my room.

But I spent big mobile data after start using Mokee OS.

My room’s situation is below.
My room’s internet service provider have some troubles.
Time to time, My room’s router lost connecting to internet.
But Wifi can keep connective in my room when router is disconnective.

My Wifi was not disconnected.
But My routers internet connection was disconnected time to time.

When room’s internet disconnected, my phone display disconnect mark and Wifi signal strength
on the status bar.
-disconnect mark : alikes ‘X’
-Wifi signal strength : midle or strong

*This situation is that only internet provider failed.
Wifi AP and local network is alive.

I think, this trouble cause is Android use mobile internet automatically whether connected to WiFi or not.
And notification and setting is nothing.
Past time old android ask us about mobile data usage ?

Android ask me about keep Wifi connection when can’t use internet.
Of course I was answering to keep the connection.
But mobile internet was used.

I think, solution is below.
-A.Android inform user changing mobile internet data.
-B.Android let user chose to continue using wifi or changing mobile internet(not turn off wifi) or turn off wifi.
-C.User set up about B’s automatically mobile data conecting. And A’s notify way.
-D.User set up about disable C each Wifi AP.