This is the flow
internet---service IP to router---home Ip service
internet-------112.23.12.33--router--192.168.1.254 or other IP used
the IPhone app knows the internet side of the router and the Cam ip so it can get to the cam from anywhere.
it is using the port forwarding the router offers.
you need to log onto the router to check the IP addresses it is using on the internet side and see if you can find the camera's ip and port it is using.
the cam will be something like this 192.168.1.99:8000
try looking at the I-phone app and see if it will show the numbers it is using.
if you do find the numbers then use a browser to see the cam
example
http://112.23.12.33:8000
you generally do not need a special app to see cameras unless the cam manufacturer has some type of lock on it or you need a password to see the view.
this may help
How to view your IP camera remotely via a web browser - TP-Link