Android 6.0 shutting down data connection (Wifi and mobile data) in standby

I am not sure if this is a 6.0 problem or the problem of my new umi super so please excuse that try finding answers here first. My Phone is shutting down mobile data and wifi connection when going to standby.. not immediatly but after an undefined time span. Calls and Calender notifications are still comming throught but i recieve a billion whatsapp messages at once when turning on the screen. any ideas? i have never used clean 6.0 just sultan roms up to know, so i cant really say if this is a basic problem, an umi problem or just settings i screwed up.

Thanks,

M