I think you mean a VPN (sorry those abbreviations are a bit confusing). Most consumer VPNs are not mean to host things. So you need to either set up a fully private one only between you and your friends (with Wireguard or similar), basically what Hamachi does, but more modern, or you need to get something like Tailscale that is specifically meant for exposing your server behind a CGNAT.
But the free tier of Tailscale will likely be insufficient, and once you pay, you might as well pay for a cloud server (VPS) and host the Luanti server there 24/7.