From here: https://blogs.technet.microsoft.com/enterprisemobility/2009/07/01/using-multiple-monitors-in-remote-desktop-session/
If your RDP's "Use all my monitors for the remote session" is disabled you can get around it as follows.
1. Create an RDP file with your specific connection.
2. Right click on the .rdp file and select Open with Notepad.
3. Add the following 2 lines to the bottom.
4. Save and then use!
Have a great day!