You’ll need to use manual port forwarding, not UPnP.
Map your Plex port to 443.
In your router, port forward external port 443 to internal port 32400.
In Settings → Remote Access, manually specify the port as 443.
Try it from an external location - coffee shop, phone on mobile network (wifi off), etc., to verify it works. Then try from work.
Also, from a PC on the same network as your server, go to canyouseeme.org. Swap the default, port 80, for your remote access port (443) and see if your Plex server is visible.
Using port 443 to access Plex from a hospital:
https://support.plex.tv/articles/categories/features/remote-access-server-sharing/
