Trying to RDP from the Internet into a workstation on the internal network on a custom port thru an inherited SG 105.

I have a new client that uses a SG 105. They previously used the Small Business Server remote workplace to remote into one station. Something in Windows 10 has broken that.  I am trying to allow RDP to a certain station on the internal network on a custom RDP port.  This is something I have done on repeatedly on Cisco ASAs and Pixes and most of the home variety routers.  Can someone point me to some information to help me figure out what to do.  

I am afraid to just start trying things because if I break something I would not know how to fix it.

I can see where my attempt to RDP is shown in gray on the Firewall log, but I cannot connect.

Any help would be appreciated.

Jeff