Mobile data stops when phone goes idle or sleep

I'm using Motorola droid turbo running marshmallow updated few months before. When my phone goes in sleep mode or idle for about 15 mins apps stop syncing on mobile data even after screen is turned back on. Then I have to turn off mobile data and then on to receive any messages/notifications.

I have tried resetting the mobile and and turning off battery optimization for WhatsApp and Facebook etc but problem is still there.

Background data is also enabled for all apps.

Battery saver mode is off.

Please suggest any remedy.