The newer the internet contract, the more likely it is that your ISP has put you behind a Carrier-Grade-NAT. Carrier-grade NAT - Wikipedia
Only your ISP can do something about that, and they do it only if they’re willing to. The incentive for them is of course money. Many will offer you a “static” public IP if you shell out more money.
(Technically, it is not necessary to have a truly “static” public IP, but that is what will bring you a publicly accessible IPv4 address.)
If you do that, make sure to stop and restart your server machine and the plex server app on it afterwards.
