The Cloud Key Gen 1 doesn't get any update via web UI anymore. To keep the controller up to date and compatible with new devices the update can be performed via SSH on the command line.

apt-get install unifi

Example:

root@UniFi-CloudKey:~# apt-get install unifi
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages will be upgraded:
  unifi
1 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 147 MB of archives.
After this operation, 22.2 MB of additional disk space will be used.
Get:1 http://www.ubnt.com/downloads/unifi/debian/ cloudkey-stable/ubiquiti unifi all 7.2.95-18699-1 [147 MB]
Fetched 147 MB in 2min 48s (873 kB/s)
Preconfiguring packages ...
(Reading database ... 26209 files and directories currently installed.)
Preparing to unpack .../unifi_7.2.95-18699-1_all.deb ...
Unpacking unifi (7.2.95-18699-1) over (6.5.54-16676-1) ...
Processing triggers for systemd (230-7~bpo8+2.ubnt+1) ...
Setting up unifi (7.2.95-18699-1) ...

It can take a couple of minutes for the controller to be available again. Wait 15 minutes before you take any troubleshooting/restore actions.

You probably want to reboot the system after you verified that the controller is up and running.

systemctl reboot

The Cloud Key uses systemd-resolved as DNS client. The change the DNS server, the file /etc/systemd/resolved.conf can be edited via SSH.

vim /etc/systemd/resolved.conf

Restart the service.

systemctl restart systemd-resolved

Check the status of systemd-resolved.

systemd-resolve --status

root@UniFi-CloudKey:~# systemd-resolve --status 
Global
          DNS Server: 192.168.1.255
          DNS Domain: int.example.org
          DNSSEC NTA: 10.in-addr.arpa
                      16.172.in-addr.arpa
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa
                      18.172.in-addr.arpa
                      19.172.in-addr.arpa
                      20.172.in-addr.arpa
                      21.172.in-addr.arpa
                      22.172.in-addr.arpa
                      23.172.in-addr.arpa
                      24.172.in-addr.arpa
                      25.172.in-addr.arpa
                      26.172.in-addr.arpa
                      27.172.in-addr.arpa
                      28.172.in-addr.arpa
                      29.172.in-addr.arpa
                      30.172.in-addr.arpa
                      31.172.in-addr.arpa
                      corp
                      home
                      internal
                      intranet
                      lan
                      local
                      private
                      test

Link 7 (eth0)
      Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: allow-downgrade
    DNSSEC supported: yes
          DNS Server: 192.168.1.255
...