In order for your router to be accessible from the Internet side, your router must have a publicly accessible (i.e. “routable”) IPv4 address.
CG-NAT does prevent this. Since routable IPv4 adresses are scarce and are traded for ever-increasing prices among ISPs, many ISPs take them away from their customers in typical residential applications, where they are typically not needed. Unless that customer wants to have a “server” of some kind – like a Plex server.
It is very likely, that your ISP has made a change, so that all their customers are now behind a Carrier-Grade NAT. So they all share one (or maybe a few) regular IPv4 adress.
It is therefore logical that your ISP is going to request additional payment if they need to give you one of the valuable IPv4 adresses just for yourself.
And this is unfortunately still needed for Plex remote access, because the IPv6 access is not fully implemented yet.
TL;DR: your ISP has likely made a change, which put you into a double-NAT situation.
