Spent a good 4 hours this Tuesday evening trying to setup up a VPN between my new Ubiquiti UDM router using the internal Radius VPN server using L2TP/Ipsec. Coming from an Asus router that had the OpenVPN wizard to work with, I had no problem initially setting up the UDM for a Radius VPN server (well, it was not super hard). However, the built in VPN client software in Android 9 & 10 (simply called "VPN") kept disconnecting after 90 seconds, on the dot. After hours of searching, it appears there is a bug in Android 9/10 specifically on the Pixel 3/3a phones using the L2TP/IPsec VPN protocol (this was fixed in Android 10 beta, but broke again in Android 10 final). Known bug for over a year. I needed to upgrade to a Pixel 4 XL anyways 
Just wanted to toss it out there for anyone related to Pixel 3, VPN, and L2TP/IPsec.

Just wanted to toss it out there for anyone related to Pixel 3, VPN, and L2TP/IPsec.