It appears your goal is to setup a VPN endpoint (server) at your home so that you can connect remotely to use your home network resources and/or the internet through your home connection. Further, the OpenWrt router that will have the VPN endpoint running sits behind another router which is responsible for your main LAN. If that is a correct interpretation, there are two ways to accomplish this goal:
- if your main router supports adding static routes, you can setup your OpenWrt router such that you're using a LAN-LAN connection (no WAN needed on the OpenWrt side). You'll add a static route from the main router so that it is aware of the WireGuard network and how to send traffic back through that tunnel.
- If your main router does not support static routes, you'll need to use NAT Masquerading on the OpenWrt router's upstream connection (i.e. the one that goes to your main router's LAN). This is the default configuration with the OpenWrt WAN, so typically LAN-WAN the easiest method (although there are nuances here, we can still use a LAN-LAN connection if you want).
Please check your main router to see if it supports the ability for you to add static routes. That will inform how we approach your configuration.