Are you trying to offer a port for peer sharing (XDCC/BT)? I’ve never tried using it like this but I think Tailscale Funnel could work.
It’s a sort of reverse VPN, I guess you could call it. Tailscale maintains the public IP and when someone connects to your advertised port they tunnel it to you through (encrypted) WireGuard. It passes through NAT because connections are outgoing to their servers.
The catch is that wireguard is easily detectable through deep packet inspection so if your ISP is a real asshole they can kill the connections, but if they go that far then NAT traversal is the least of your worries.