Installation guide for several Linux systems: https://community.ui.com/questions/UniFi-Installation-Scripts-or-UniFi-Easy-Update-Script-or-UniFi-Lets-Encrypt-or-Ubuntu-16-04-18-04-/ccbc7530-dd61-40a7-82ec-22b17f027776

Download Ubuntu for RPi (64bit version for RPi 3) image at http://cdimage.ubuntu.com/releases/20.04/release/ubuntu-20.04-preinstalled-server-arm64+raspi.img.xz

Put SD-Card for RPi in laptop and run following command on laptop:

  xz -cd ubuntu-20.04-preinstalled-server-arm64+raspi.img.xz | sudo dd of=/dev/mmcblk0 bs=1024

Boot RPi and follow post installation steps. To login for the first time via ssh takes a good while - a couple of minutes. Afterwards let the system run its unattended upgrade process. This takes a loooong while.

Afterwards manually run apt update && apt upgrade. This takes a loooong while again.

Reboot the RPi.

Login again on the RPi and become root using sudo su and remove some stuff you won't need:

  apt purge snapd
  apt purge wpasupplicant

Download the install script

  wget https://get.glennr.nl/unifi/install/unifi-5.13.29.sh 

(Alternatively, it is here too: unifi-5.13.29.sh.zip)

Run the script:

  chmod 755 unifi-5.13.29.sh
  ./unifi-5.13.29.sh

Answer all questions the script asks with yes.

That's it.