^ What meandu229 said. ^
Whatever address you used for connecting to your computer via your local network is the one that port 5900 should be forwarded to. Was it 192.168.1.107? (Although if you are using your router's DHCP feature, then this could also change. You should probably give your Windows computer a static IP address if you want to do this regularly.)
An alternative way of seeing your router's external IP address is to look at its configuration; you should find some kind of status page that will tell you what IP address was assigned by your ISP. The internal IP address of your router is probably 192.168.1.1 (although it could be anything on the 192.168.1.x subnet). You want to use its other address not starting with 192.168.
Also, I'm not that familiar with Remmina, but on conventional VNC clients any number following a colon in the IP address is the display number, not the port. In other words, if you try to connect with 192.168.1.107:5900 (just an example, since I don't know your actual external IP address, and you shouldn't post it), then you are trying to connect with display number 5900 on the VNC server (which almost certainly doesn't exist). Windows VNC servers generally serve only one display, numbered 0, which is the default display number that clients try to connect to. In actuality, the number you use does denote a port number, but the actual port number is whatever number you give plus 5900. In other words, if you specify 0, then it is port 5900, if you specify 1, then it will be port 5901, etc. So if you specify 5900, then it may be 11800 (I have never seen a port higher than 3 actually in use, so I'm not sure if numbers that high actually work or not).