rd web access and dual monitors

Jim_Satterfield   Every time you use the Remote Web portal to your server and click to initiate a RDP session, it downloads an RDP file to your default downloads folder, then runs it. 

It's entirely valid to save the most recent RDP and re-use it indefinitely and skip the web portal for future sessions.   This allows you to use custom settings right at your client level - instead of relying on the settings as defined by the Server 2016 portal settings (which IMO are a bit wonky.)

Once you've saved off that RDP file, you can right click it and select Edit.  Go to the Display tab and UNselect "Use all my monitors" then Re-select it, also make sure the slider is set to use Full Screen, then click back to the General tab and save.  Use that RDP file for all future sessions once you get the settings right for your needs.

If you have more than two monitors, there is a "secret method" to enable more (if that's what you want) or even force the use of a sub-set of your available monitors.   See this write-up from Scott Hanselman for details on how to make that work:

https://www.hanselman.com/blog/HowToRemoteDesktopFullscreenRDPWithJustSOMEOfYourMultipleMonitors.aspx

Hope this helps!