Why can I not connect to my Bedrock server from outside?

I have a Bedrock dedicated server set up in my home NAS, and I can access it just fine on my local network (LAN). I've forwarded ports 19131-19133 through my router to the NAS, and I was able to get a ping from the internet to my server (on port 19132). However, whenever I try to connect externally, it always fails with "Could not connect to world."

I did notice in the logs, whenever it starts up it indicates two different ports:

[INFO] IPv4 supported, port: 19132
[INFO] IPv4 supported, port: 39187 // <-- changes every time

I've tried looking online for answers, but no joy. For reference, I'm using marctv/minecraft-bedrock-server on Docker in my Synology. Not sure if that helps.

edit: mistyped port range.