Have you configured your AT&T router/modem to allow for that kind of external port forwarding? Usually going with a manual port forward tends to be more stable/reliable – but UPnP can also work.
You might want to take a look at this guide on troubleshooting your remote access. Any findings from that will help pin down the problem you’re experiencing.
As a side note:
The remote access status indicator displaying a green icon and then switching to red doesn’t mean remote access starts ok and then fails. This is based on how Plex detects the remote access status in different steps and only turning it to “red” after finishing all the steps (and failing in the end).
