Sounds like a firewall or NAT issue. Assuming you're using the correct public ip address (your outside, or external one given by the ISP) theres likely a problem with the ip:port combo in that either the firewall is blocking it or the router is not sure where to forward it
Have you tried disabling windows firewall or making sure the proper exception has been made for VNC?
Are you sure the port the VNC program on your phone is the correct one forwarded? Almost any time i've ever forwarded a range of ports there has always been an issue for something. Just never seems to work for me. Specific ports always seem to work though.
Also the type of forwarding may be an issue as well. If the forwarding only forwards TCP and not UDP, or vice versa depending on what the program uses, that can cause a networking issue. (same thing with windows firewall)
Hope this helps if you haven't already resolved your issue. If not and you still need help... I have an android phone and would be behind my own set of routers and firewalls, so I could probably figure out what needs to be forwarded. But i'm lazy and its been two days since you posted this so lemmy know