The way I used to do it on my HH5 was to first use a DDNS service - I used NoIP - just have to confirm your details every 30 days.
Then go into port forwarding and create a rule for each camera - you should be able to enter IP address of camera required and port number you want to use (make sure you have set static IP address and port number you want to use in camera set up).
You should be good to go - This set up may have changed a bit as have now got rid of the HH5 and use a Draytek which works a lot better than the BT Hub!!!