Connect your iOS device to wi-fi network.
Go to "Settings" › "DLNA" › "Remote Access" and press "Add" button.
Press "Copy info from the listed DLNA server" and select your server.
Replace IP in the URL field with your external IP and press "Save".
Open original URL in browser.
Forward port from the URL.
Open new URL with external IP address. If you forwarded ports correctly the same page should appear.
Open any folder with media items on your DLNA server.
Use "Long press" for any media item and select "Diagnostics" in pop-up menu.
Find your IP address in "Diagnostics" info. You’ll see the port which your server uses for media items.
If your server uses another port you need to route it as well.