Unifi Controller

Debian 9

apt update && apt install ca-certificates apt-transport-https
echo 'deb https://www.ui.com/downloads/unifi/debian stable ubiquiti' | tee /etc/apt/sources.list.d/100-ubnt-unifi.list
sudo wget -O /etc/apt/trusted.gpg.d/unifi-repo.gpg https://dl.ui.com/unifi/unifi-repo.gpg
apt update && apt install unifi

Debian 10

This is just recording steps I took, may not work. In my case, the Unifi service would start but the web interface would never load. Netstat would show that the needed web ports were not listening.
On clean install

• Basic packages needed

apt update
apt upgrade
apt install wget curl libcurl4

• MongoDB Packages

apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 9DA31620334BD75D9DCB49F368818C72E52529D4
echo "deb http://repo.mongodb.org/apt/debian stretch/mongodb-org/4.0 main" | tee /etc/apt/sources.list.d/mongodb-org.list
echo "deb http://deb.debian.org/debian/ stretch main" | tee /etc/apt/sources.list.d/debian-stretch.list
apt update
apt install mongodb-org mongodb-org-server libcurl3

• Install dependencies.

apt install binutils curl openjdk-8-jre-headless jsvc libcurl4

• Edit Unifi package and install

wget https://dl.ui.com/unifi/5.12.35/unifi_sysvinit_all.deb
mkdir tmp
dpkg-deb -R unifi_sysvinit_all.deb tmp
sed -i '/mongodb-server/ d' tmp/DEBIAN/control
dpkg-deb -b tmp unifi-fixed.deb
dpkg -i unifi-fixed.deb