RDP with port forwarding

I want to remote on to a windows machine from outside my network with Ubuntu. I use DynDNS to track the ip address of my router. The router is set up to use port forwarding. From a Windows machine I can use companyname.ath.cx:PortNumber. But I can't get this to work from Remmina. I have tried using my current router ip and the port number (e.g. 123.456.789.11:1234) but no luck.

Any ideas please. Thanks - Peter

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