Assuming that you are using “firewalld” then the config program is “firewall-config”, which I don’t think is in YaST2. The applet in YaST2 pointed to the old SUSE YaST2 firewall so I got rid of it. That might have been a hangover as my system has had a long history of upgrades.
Anyway. I just enable the “rdp” service in “firewall-config” (alternatively from the KDE menu system -> firewall).
All just worked for me and I use a variety of clients.