Problem:
After your system has started, you need a script to run always.
Solution:
On a typical Linux system you can simply created an /etc/rc.local file and add some BASH commands, but on Debian 9 rc.local has been deprecated.
The solution on how to get it back is listed in this article:
http://www.itechlounge.net/2017/10/linux-how-to-add-rc-local-in-debian-9/