Lists of ports I need to allow to be able to use plex properly:

TCP: 32400 (access to the Plex Media Server) [required]

The following additional ports are also used within the local network for different services:

UDP: 1900 (access to the Plex DLNA Server)
UDP: 5353 (older Bonjour/Avahi network discovery)
TCP: 8324 (controlling Plex for Roku via Plex Companion)
UDP: 32410, 32412, 32413, 32414 (current GDM network discovery)
TCP: 32469 (access to the Plex DLNA Server)