IPSEC is an aging protocol, and in many ways, is inferior to some of the more current VPN protocols. There is no official wiki page explaining how to configure it.

However, these unofficial

Sources:
https://www.linksysinfo.org/index.php?threads/tomato-and-ipsec-why-not.38344
https://gist.github.com/dferg/50c378701a192bc8ce6a (Requires a Github account).
https://openlinksys.info/forum/viewthread.php?thread_id=20790 (In Polish. Google translator may help).