Run the following commands on your UniFi Cloud Key:

NOTE: This guide is outdated. See latest offical support for UniFi devices:

https://docs.zerotier.com/devices/ubiquiti/

curl -s https://install.zerotier.com | bash

Output:

*** ZeroTier One Quick Install for Unix-like Systems

*** Tested distributions and architectures:
***   MacOS (10.7+) on x86_64 (just installs ZeroTier One.pkg)
***   Debian (7+) on x86_64, x86, arm, and arm64
***   RedHat/CentOS (6+) on x86_64 and x86
***   Fedora (16+) on x86_64 and x86
***   SuSE (12+) on x86_64 and x86
***   Mint (18+) on x86_64, x86, arm, and arm64

*** Please report problems to [email protected] and we will try to fix.

*** Detecting Linux Distribution

*** Found Debian "jessie" (or similar), creating /etc/apt/sources.list.d/zerotier.list
OK

*** Installing zerotier-one package...
Hit https://deb.nodesource.com jessie InRelease                                                                                                  
Hit https://deb.nodesource.com jessie/main armhf Packages                                                                                                                                              
Get:1 http://download.zerotier.com jessie InRelease [20.9 kB]                                              
Ign http://httpredir.debian.org jessie InRelease                                                                                
Get:2 http://security.debian.org jessie/updates InRelease [44.9 kB]                                                   
Hit http://www.ubnt.com cloudkey-stable InRelease                                           
Hit http://www.ubnt.com cloudkey-stable/ubiquiti armhf Packages                                                     
Hit http://httpredir.debian.org jessie Release.gpg                                   
Get:3 http://download.zerotier.com jessie/main armhf Packages [2479 B]
Hit http://httpredir.debian.org jessie Release                                          
Hit http://httpredir.debian.org jessie/main armhf Packages                                                                                                                                             
Hit http://httpredir.debian.org jessie/contrib armhf Packages                                                                                                                                          
Hit http://httpredir.debian.org jessie/non-free armhf Packages                                                                                                                                         
Get:4 http://security.debian.org jessie/updates/main armhf Packages [695 kB]                                                                                                                           
Hit http://security.debian.org jessie/updates/contrib armhf Packages                                                                                                                                   
Hit http://security.debian.org jessie/updates/non-free armhf Packages                                                                                                                                  
Fetched 764 kB in 1s (764 KB/s)                                                                                                                                                                  
Reading package lists... Done
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following NEW packages will be installed:
  zerotier-one
0 upgraded, 1 newly installed, 0 to remove and 2 not upgraded.
Need to get 763 kB of archives.
After this operation, 2366 kB of additional disk space will be used.
Get:1 http://download.zerotier.com/debian/jessie/ jessie/main zerotier-one armhf 1.4.6 [763 kB]
Fetched 763 kB in 44s (17.3 kB/s)                                                                                                                                                                      
Selecting previously unselected package zerotier-one.
(Reading database ... 23329 files and directories currently installed.)
Preparing to unpack .../zerotier-one_1.4.6_armhf.deb ...
Unpacking zerotier-one (1.4.6) ...
Processing triggers for systemd (230-7~bpo8+2.ubnt+1) ...
Setting up zerotier-one (1.4.6) ...
Processing triggers for systemd (230-7~bpo8+2.ubnt+1) ...

*** Enabling and starting zerotier-one service...
Synchronizing state of zerotier-one.service with SysV service script with /lib/systemd/systemd-sysv-install.
Executing: /lib/systemd/systemd-sysv-install enable zerotier-one

*** Waiting for identity generation...

*** Success! You are ZeroTier address [ e50a7c7fc2 ].

Run zerotier-cli to see if everything works:

zerotier-cli 
ZeroTier One version 1.4.6 build 0 (platform 1 arch 3)
Copyright (c) 2019 ZeroTier, Inc.
Licensed under the ZeroTier BSL 1.1 (see LICENSE.txt)
Usage: zerotier-cli [-switches] <command/path> [<args>]

Please note after upgrading the Cloud Key firmware, you need to reinstall the Zerotier One package. And if you got the following error when trying to reinstall the package:

*** Enabling and starting zerotier-one service...
Synchronizing state of zerotier-one.service with SysV service script with /lib/systemd/systemd-sysv-install.
Executing: /lib/systemd/systemd-sysv-install enable zerotier-one

*** Waiting for identity generation...

Press control – C and execute the following to restart the service and rejoin the Zerotier network:

systemctl restart zerotier-one
systemctl status zerotier-one
zerotier-cli info
200 info 8ade3af9ef 1.4.6 ONLINE
zerotier-cli join eda9f5dbfe94adbe
200 join OK
# ...then approve it in your Zerotier dashboard
# list network
zerotier-cli listnetworks -j