There are lots of guides on how to do this with OpenVPN, but I wanted to use the built in client in Android to achieve a stable L2TP connection. Works well on my Chromebook as well!
Here’s a great guide I found, specifically for the Raspberry PI, but should work on Debian based distros as well, if you omit the first part…
https://www.stewright.me/2018/07/create-a-raspberry-pi-vpn-server-using-l2tpipsec/