Obligatory: new to proxmox, and trying to understand networking.
TL;DR: container needs to look like it has:
IP = 12.34.56.778 /28 with VLANid=1000,
gateway = 12.34.56.777 to the pfsense router for unfiltered internet access (plex server).
Simplified setup:
internet > pfsense > proxmox...