Yes plex can be setup for remote viewing, i believe this does require paying for plex pass or plex viewer pass.
alternatively there are other more involved but free options for setting up free remote viewing as well.
by default you will be confined to your local network only.
you will have to use one of the more involved solutions mentioned above.
there are cloudflare tunnels, tailscale and other options for getting this to work for you.
We do not recommend using external drives however you can setup external drives to work that way.
Most likely you will need to wipe the drives as HexOS.