When I changed to a USG from my Edgerouter, all VPN's stopped working. Contacted support, and read all the posts, but could not get it functioning. My solution (hope it helps others) was simply creating longer passwords for the Radius users in Unifi/USG. All the setups were configured with copy/paste so I am certain that all the information was set up correctly all along, but changing the Radius password to longer (I am currently using one at 11 characters, and one at 10 characters) resulted in immediate connectivity from all clients, Android, Windows 10, iPhone.
Hope this helps others, let me know if you find this to be true in your challanges, if so perhaps it can be documented for others.