If your theme is not responding to mobile devices then it could be an issue with your cache settings. Or the plugin itself.
Novapress Pro is definitely responsive.
I have not tested the theme with the WP Fastest Cache plugin, so I can not guarantee 100% compatibility.
My other recommendation to clear you cache manually after you have activated the plugin. I hope this will solve your problem.
If this does not solve the issue for you, I sadly don’t have a solution for you at the moment.
I might be able to do some basic testing with WP Fastest Cache in the future, but I can not give any guarantees.