Royal TS is using the RDP ActiveX control which, by default, doesn't work correctly when running it on Windows with high DPI settings (under Control Panel -> Appearance and Personalization -> Display).
In this case you need to configure Royal TS to disable display scaling:
  • Close Royal TS.
  • Open the Royal TS shortcut or application Properties dialog in Windows Explorer.
  • Select the Compatibility tab.
  • Check the option " Disable display scaling on high DPI settings ".
  • Start Royal TS.