Not familiar with cloudflare. But generally speaking, purchase and register a domain and point the DNS to cloudflare. Configure the A record of your domain to your home’s public IP address. In your firewall you want to set up a translation rule to send any traffic on a port you assign externally to whatever IP and port your nginx server is internally.

You can also consider not exposing any of your services publicly - keeping your home server more secure. Instead set up a wireguard (or other) VPN, expose just that and point your domain or subdomain to it. Then, while you’re away, connect to your VPN and have your audiobookshelf client on your phone connect to you lan IP address /port.