IIRC, FiOS has network security settings that can interfere with Plex remote access. You may need to disable them in your FiOS account and/or router settings.
Run through the steps in this document: The Basics of Remote Access Troubleshooting
Ensure the Plex Server network interface is Private, not Public (Windows Settings → Network & Internet → Properties).
Verify the Plex server IP address & default gateway are correct and match the port forward statement in the router.
Configure the Plex server to use a public DNS server such as 1.1.1.1 (Cloudflare), 8.8.8.8 (Google), or 9.9.9.9 (Quad9).
If you change any network settings on either the router or the Plex server, then reboot the router and when it is back online, reboot the server.
Plex remote access is never working. Plex “optimistically” shows remote access as working while it is trying to initialize things. When that fails, it returns to “not available…”
You phone may be connecting via Plex Relay, a backup system used when remote access is not available. The stream is tunneled through servers at plex.tv and limited to 2 Mbps. Plex Relay is enabled/disabled in Settings → Network.
