No, you misunderstand. You’re thinking of DHCP. The parent poster is talking about CGNAT, where hundreds or thousands of customers of an ISP may share the same public-facing IPv4 address. It’s impossible to self-host anything in this scenario, there no way around it and DDNS won’t help you.
I don’t remember being confused. But it was definitely one of those movies where I had to sit for 15 minutes after the credits rolled to process the emotional impact.