FreeRDP works great but if I try to RDP to a legacy Windows NT OS there is no way to send "Alt+Ctrl+Del" key sequence to initiate a login using FreeRDP.
I have not found any native OS/2 client that has that feature.
What I do now is use FreeRDP to my Raspberry PI running Ubuntu and use Remmina to remote desktop to Windows NT type OS since it does have send-key.