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".