in order to access this through your firewall, you can use a router that can redirect to a different port (so your router would take http://your_external_ip:81 and send the traffic to http://your_internal_dietpi_ip:80)
or you can reconfigure Apache to listen on a different port (such as 81 or 8080)… Here’s the basics, but you may need to Google for more details
You can use rhkean’s suggestion along with noip. Follow the instructions here: http://dietpi.com/phpbb/viewtopic.php?f=8&t=5&start=10#p58
Make sure to enable port forwading on your router and that should be it. Whether its local or online, you will be able to reach your ownlcoud from anywhere.