NVidia Shield server running but not visible

Server Version#: v1.19.5.3112
Player Version#: web app/phone app

I am unable to see the plex server on my NVidia Shield when connecting over the Web, but can see it when I am on the same network. The plex web app says that it is available over the network:

Fully accessible outside your network
You can access this server from signed-in Plex apps or in a browser at https://plex.tv/web.
Private 192.168.8.102 : 32400 Public 2.143.52.154 : 0 Internet

However, the plex app on my phone says the server is offline.

The server reports to be running. I have restarted both my network and my shield (full power-off, power-on for both). I have logged out and in again… nothing. Other apps on the Shield are fine, so it can certainly see the Internet.

It was working up until recently… then for whatever reason, it just stopped! No changes that I am aware of… it just stopped working (this has happened in the past, and the power cycling fixed it… but not this time…)

Are there troubleshooting steps I can go through?

thanks!