So herein lies a challenge. It is that just because the VPN
server is responding on TCP port 443 does not necessarily
mean that everything is correct.
In other words, you could run this test.
It could come back as positive or as valid.
But if there's a configuration error, the VPN still may fail to connect.
For example,
some common failure scenarios that I see are that the NAT
rule is configured incorrectly and it's sending the traffic
to just a regular web server.
Or I have also seen where the management interface,
which is an HTTPS management interface on the firewall,
is actually responding instead of the VPN server.
And there's a good way to test that using PowerShell.
And to do that, what you'll need to do is run this command.
It's Invoke‑Web‑Request,
and you'll see that this is a rather complex command here.
But bottom line is copy this command. And if you'll
notice that I'm using vpn.example.net,
you'll need to replace this with your public host name.
But the rest of that path should be exactly as it's defined here,
and make sure you use basic parsing as well.
So when you run this command, I'm just going to paste it in here.
When you run this command and hit Enter, what you
should see is an error 401 Unauthorized.
This would give you a good indication that the VPN server is
actually responding because that's the expected response when you
just simply send an HTTP GET to the server.
So if it returns something else, and, for example,
let's say it was the NAT rule forwarded the traffic to like a
common web server and not the VPN server, again,
Test‑NetConnection will report TCP port responding,
but you will also get a very different response.
And what you'll see here is this server returns a 503 Unavailable.
This would be a strong indication that the correct resource
is not actually responding because, again,
the VPN server is going to respond with a 401
Unauthorized, any other normal web service.
Or if it's the firewall's web management console,
it will respond with something else, and in this case a 503. That
tells me I still need to investigate the networking and make sure
that my firewall is configured correctly and delivering the traffic to the VPN server as expected.