Check whatismyipaddress.com to see your IP address once you’re connected to either network, but with a high likelihood, it’s almost certainly different IPs. In that case, Dynamic DNS is probably best.
But if you’re using your neighbor’s wifi, I doubt there’s a way for you to host stuff unless you have access to their routers, can open ports 80 (HTTP) and 443 (HTTPS), and forward them to your server. It’s best to use hardware you control (including the router).