Plex Remote Access cannot connect to Oracle Cloud Compute

Server Version#: 1.18.4.2164-2aa83397b

I’m trying to set up a Plex Media Server on a Oracle Cloud Compute instance, but the remote access never works. I’ve opened port 32400 tcp as ingress rule on Oracle Cloud console, disabled ufw, and netstat shows Plex is listening on tcp6 port 32400, but yet the remote access never work. It just keep showing cannot be accessed from Internet. Even when trying to use user defined port. When I press retry, after awhile it shows accessible, then after one second it goes back to unable to access.

Does anyone have any experience on doing this on Oracle Cloud Compute?

OS is Ubuntu 18.04

Do they allow UPNP / inbound connections to your system on other than the standard 80/443 ports?