You have DS-Lite, so it’s the usual story like with all the other millions of users with that:
- connectivity over IPv4 is through CG-NAT, so no direct access
- connectivity over IPv6 technically works, just open port 32400 in the router firewall. You’ll have to set it up manually with Plex (own domain name + cert), and of course the clients also need to be on IPv6.
If you can get your ISP to move you back to single stack IPv4 with a public ip address, that’s a workaround until Plex has added automatic IPv6 support. To speed that up (it’s been seven years now), vote for this: https://forums.plex.tv/t/ipv6-support-for-myplex/
