How can I open some ports on Ubuntu?

I know a little about Linux. Today I created a [VPN][1] server on my Ubuntu installation according to *[Set up a simple IPsec/L2TP VPN server for Ubuntu, Arch Linux and Debian][2]*. But when I finish the installation, I use my iPhone to connect the [IPsec][3] VPN, bur it shows the VPN Server has no response. The GitHub document shows > Ports 1701, 500 and 4500 must be opened for the VPN to work! So I have tried to open these ports on my Ubuntu server. I use the [iptables][4] command to open these ports, but it failed. Maybe I don't known how to use this command correctly. How can I open these ports on my Ubuntu server? And if these ports have been opened successfully, can it be proved by the Windows [CMD][5] window through [telnet][6]'ting the port? [1]: https://en.wikipedia.org/wiki/Virtual_private_network [2]: https://github.com/philplckthun/setup-simple-ipsec-l2tp-vpn [3]: https://en.wikipedia.org/wiki/IPsec [4]: https://en.wikipedia.org/wiki/Iptables [5]: https://en.wikipedia.org/wiki/Cmd.exe [6]: https://en.wikipedia.org/wiki/Telnet