Unable to set up VPN server
I am unable to properly set up and access a VPN server. Help is much appreciated.
Setup
Server: 2009 MBP, 15", 8 GB ram; OSX 10.10.5 ; Server.app 4.1.5.
Client: 2015 MBP retina, 13"; OSX 10.10.5
Router: 2009 Time Capsule (firmware 7.6.4)
Using Airport Utility (6.3.5), I have given the server a reserved IP and forwarded ports 500, 1701, 1723 and 4500 to that IP (and also 22 for SSH).
I have set up a DDNS for the server (using MAC address of wireless card, which I am using for now).
Using Server.app I have set up the VPN to allow only my user access. There's a green dot on the Services>VPN item in the sidebar, but in Server><computer_name> in Overview:Server:Internet it says Reachable at 87.––.––.––. (–– is my redaction), no services available.
With the url for my DDNS, I can log in to the server using ssh, which I take to mean that I am on the right track.
But when trying to connect to the VPN (having set the same URL as hostname), I get no response from the server and hence no connection.
I have tried verifying the port openness using http://www.yougetsignal.com/, but this is apparently unable to reach any port, perhaps this is a clue.
I don't really have any prior networking experience. What could I be doing wrong?