Hello,
I have an Amcrest IP2M-841 ProHD 1080P camera. I have it setup for port forwarding, and am using the Amcrest View Pro app on my iPhone. I setup the camera in the app to connect to the external IP of the home network. If my phone is using cellular data, I am able to access the camera's feed and remote control it with the app.
However, if I join a WiFi network, what appears to be any WiFi network outside the home network, I get a "Login timed out" error when I try to access the camera. If I access the HTTP port of the camera from a computer on the same remote WiFi network that my phone is on when getting the error, I am able to successfully log in to the camera's UI.