Just trying to help out here though it’s worth confirming that your MineOS server can actually reach the internet ? If you were missing / mis configured your Default Gateway on your MineOS server then it would not be able to reply to a request coming from a different network ?
My first port of call is to rule out the obvious. You say that it works locally when a client is connecting on the LAN, so you wouldn’t really need / use your default gateway in this particular instance.
If your setting up port forwarding to a different machine then logic would state your ok with setting up port forwarding. I would then look to try and install ‘TCPDUMP’ on your MineOS server and see if your are receiving any requests from the internet.
As others have stated, it wouldn’t be remiss to rule out local machine firewalls so its worth turning this off.
Finally, you could perform a test and put your port group into Promiscuous Mode. It would rule out any Vlan policy issues that may or may not be present on a vSwitch.
Further info on that can be found here:
You could also try to telnet to see if that produces a response on your terminal as that will indicate whether your minecraft server is listening on the correct socket…
e.g - telnet x.x.x.x 25565