If on your LAN you use the LAN IP.
If from the web you use your router's EXTERNAL IP.
I didn't write about port forward, firewalls in router/PC and such because your VPN setup pages will cover that.
I am trying to set up a VPN between my iOS and my windows 10 PC using my PC as the server and when im in network set up it asks me for my server name or IP address. Do i use the IP for my PC or my router?