thanks for the reply, so the vpn tunnel doesn't block any ports from being open?
Indeed, that is a task for a firewall. (either hardware or software one)

Port forwarding is done in the router, not in a firewall.
Port opening/closing is done in a firewall.

Why do you want to use port 400 ?
That is not exactly a standard port.

What is saying that when you use the vpn the port is closed ?

What application is it you are talking about ?

Note:
You can’t test what ports are open/closed from the system itself.
You need to do it from the outside.