I have been able to successfully use L2TP vpn with UniFi firewalls using a configuration profile I created in Apple Configurator 2. Two separate configuration profiles, installed in system preferences, and it connected right up. If I add it through System Preferences manually, I get a PPP configuration error or an L2TP server did not respond. I'm guessing through system preferences, the manual installation isn't saving correctly, but using a configuration profile works great. Fingers crossed it keeps working!