It usually won’t require Plex to actively/maliciously block something… there’s already a lot of potential breaking points when it comes to remote access.
You could e.g. start checking if you can even see the router’s public IP address on the specified public port (from your port forward) outside your home network. You can do that using a service like canyouseeme.org.
If that fails it implies the issue is between your router and ISP – you could e.g. try a different public port to see if that particular one might be blocked by some party.
