Can't connect to specific IP address and/or TCP port - can the ISP be blocking it?
Solution 1:
Old topic, but I want to warn future readers that presenting RDP directly to the Internet - even on a nonstandard port - is an incredibly bad idea.
Set up a secure VPN from the user’s laptop to a network or to a specific machine in their workplace and run RDP traffic through that encrypted tunnel. This solves the possible issue of RDP traffic being intercepted by the ISP, and it works around possible weaknesses in the RDP protocol security.