Unifi Controller on Debian 9

How to install Unifi Controller 5.10.20 on Debian 9.8 “Stretch”

Used URL Links:

Download Debian 9:
https://www.debian.org

Command to install ‘dirmngr’ on debian 9:
apt-get install dirmngr -y

UniFi – How to Install and Update via APT on Debian or Ubuntu :
https://help.ubnt.com/hc/en-us/articles/220066768-UniFi-How-to-Install-Update-via-APT-on-Debian-or-Ubuntu#STEPS