Andrews and Arnold (AAISP) L2TP tunnel setup on OpenWrt
This is a guide for setting up the L2TP service from AAISP on an OpenWrt router. AAISP have an official OpenWrt guide for this but it is outdated and looks to have been originally written for OpenWrt 15.01/15.05 (Chaos Calmer), which is a long unsupported firmware build.
I have previously contacted AAISP about their documentation being out of date, however it has not been changed. It is technically not wrong, but under newer versions of OpenWrt you can simplify some of the configuration compared to the official documentation and there are also a few potential issues you might come across in OpenWrt land which are worth mentioning and saving someone else from bashing their head against a wall for hours, until you realise that the Linux kernel just hates you.
This has been tested on the latest stable OpenWrt 19.07 release branch.