First off, the scenario I deployed is on a Linux machine sitting behind a home router (NAT box). The machine we're doing the setup will be called tux.
Clients ---- Internet ---- ISP ---- Router(NAT) ---- tux(Strongswan)
For this to work port 500 and 4500 on your Router(NAT) should be forwarded to tux.
If you don't have a static IP with yout provider, you should consider a dynamic DNS service that will update the DNS address every day, when the address changes.
NOTE: I preferred to build strongswan from source to get the newest algorithms and Ed25519 curve support instead of standard EC curves. Safecurves