How to keep APP running in the backround Android 7.0 ???

[Android Central Question]

Vodafone Smart N8 (VFD-610)
Android 7.0
App NoRoot Firewall .....I tried everything (battery optimization-not optimized, unrestricted data usage for this App, I locked the lock in the recent apps menu .. nothing works

 

[belodion]

What app?

 

[feribacii]

App NoRoot Firewall

 

[ManiacJoe]

This one?
https://play.google.com/store/apps/details?id=app.greyshirts.firewall

If so, since the app has not been updated since Nov 2014, it would seem that the developer has abandoned it. You may need to switch to a different firewall app.