Background Apps in Nougat

Hello,
I have a problem. My phone's OEM released the Android Nougat update for my smartphone last week. After the upgrade however, I find that some apps which I need running in the background get killed as soon as the screen turns off. It affects music playback as well as Opera Max which I use to keep my phone's data usage to a minimum. This is really bothering me. I need help.

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