The status briefly showing as green before switching to red just means it’s not working at all. This is due to Plex‘s „optimistic approach“ to show the status as green while it’s testing the connection (this will usually take a few seconds; there’s situations where it can take up to 2-3 minutes).
Have you confirmed your port forward is still accessible from outside your home network? If it’s not, you could try switching to a different port. It might be worth to test some random ports ranging from 20000 to 50000, or a standard port like 80 or 8080 — this way you could test if your ISP might be blocking that particular port.
