How to open some ports on Ubuntu?

I know little about Linux.Today I create a VPN server on My ubuntu according to https://github.com/philplckthun/setup-simple-ipsec-l2tp-vpn But when I finish the installation. I use my iphone to connect the ipsect 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 “iptables” command to open these ports.but it failed.Maybe I don’t known how to use this command correctly。So I want to know how can I open this ports on my ubuntu server? And if this ports have been opened successfully, can it be proved by the windows cmd window through telnet the port?

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Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)