Remote access Mac behind a router with Windows PC

My Mac Mini at home is behind a router. I need to access it occasionally from work using a Windows laptop. I use the following two links to set up the connection. [YouTube: How to Access Your Mac over Internet](https://www.youtube.com/watch?v=sBjRAK0rFvk) and [Setting up Remote Access (Mac)](https://www.dynu.com/Resources/Tutorials/DynamicDNS/RemoteDesktopMac) . I enabled Screen sharing instead of Remote Management mentioned in the second link. Then I tested it at home on the LAN. First, I entered UltraVNC on my Windows laptop as the internal IP address for Mac Mini, I can access Mac Mini using UltraVNC viewer without a problem. Then I set up a port forward for port 5900 in my router and checked it with [Dynu: Port Check](https://www.dynu.com/networktools/portcheck) . The port check result is **success**. According to my understanding, that means an outside computer should be able to use the router IP address and port 5900 to send access requests to Mac Mini behind the router. However, when I enter in the software UltraVNC Viewer (still at home) either the router IP address or the subdomain created at Dynu, UltraVNC viewer reports "Failed to connect to server". Should I try outside the home? What else do I miss here?