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Two routers, one headache: double NAT
If your provider's box has its own Wi-Fi and you plugged your own router into it, your traffic crosses two routers. A forwarding rule on one carries the knock only as far as the other, where it dies. Plex stays unreachable and nothing tells you why.
How to tell
Look up your router's internet address on its status page. If it starts with 192.168, 10., or 172.16 through 172.31, there is another router in front of it. (If it starts with 100.64 through 100.127, that is CGNAT, a different guide.)
Three fixes, best first
1
Bridge the provider's boxLook for bridge mode or modem mode in its settings, or ask your provider to enable it. The box becomes a plain modem and your own router does all the routing. Cleanest fix by far.
2
Forward the port twiceOn the provider's box: forward 32400 to your router's WAN address. On your router: forward 32400 to the Plex machine. Reserve addresses on both, or it breaks when either one shifts.
3
DMZ your routerOn the provider's box, point its DMZ at your router's WAN address. Everything passes through to your router, which still firewalls normally.
Double NAT shows RFC 1918 space on your router's WAN; CGNAT shows 100.64.0.0/10. The fix paths differ, so check which one you actually have before touching anything.