In order to use the new Microsoft Remote Desktop Client (RDC) within Royal TS, the client has to be manually downloaded via the following link:
https://learn.microsoft.com/en-us/azure/virtual-desktop/users/connect-windows
Afterward, run the setup and make sure the rdclientax.dll is located in the %ProgramFiles%\Remote Desktop folder.
Now please run Royal TS V7, go to "File > Plugins> Remote Desktop > Remote Desktop Protocol Connection (based on Microsoft RDP ActiveX)" and press on "Plugin Settings" as shown here:
Lastly, make sure that the "Use Microsoft Remote Desktop Client" checkbox is activated, confirm by pressing OK and restart Royal TS V7.
That's it.
If you open an RDP connection within Royal TS and the RDP (based on MS RDP ActiveX) is your default plugin, the Microsoft Remote Desktop Client will be used to establish the connection.