The recommended way it to get a vpn at home that you can connect to when out. The your phone, when out, is operating inside your home network.
You can port forward and allow access in from the internet but that is not recommended as it a security hole. A vpn, while that is also a point of entry to your network, is generally more secure if it is set up correctly.
An easy way buy a router that has built in OpenVPN server like an ASUS Branded. On my iPhone I then just install OpenVPN app and click connect and just open DMSS to view.