There’s no clear instruction on how to allow this via the Unifi Controller. On my UDM running 1.5.6.2150 the solution is to Accept ICMPv6 and IPv6-ICMP protocols on the WAN-Local interface of the Firewall. This allows you to get 10/10 from the test site http://test-ipv6.com. If you’re using this test https://ipv6-test.com the results are different on Safari, Chrome and Firefox for me. Many comments say the ICMP test is broken and not to worry if you can’t get 20/20. Here is an external ping service to test whether you are reachable.
Here’s what the settings look like on the new user interface
Here’s what 20/20 result looks like:
