This instructions will help you to fix app background work on some devices.
More information you can find on https://dontkillmyapp.com/
1.
2.
3.
4.
5.
1.
2.
3.
4.
If you don't have previous settings try this:
3.
4.
5.
1.
2.
3.
4.
1.
2.
3.
4.
5.
Go to settings > battery > manage apps’ battery usage
2.