Failure to setup telemetry radio

I am trying (unsuccessfully) to setup a pair of telemetry radios (3DR V2.0) using the latest version of Mission Planner (v 1.3.56 Build 1.3.6672.30243).

I have physically connected a radio to the Pixhawk and my groundstation. They appear to know about each other - they show a solid green. The radio connected to Telem1 on the Pixhawk (2.4.8) also shows a flickering red led which I interpret to indicate its transmitting information.

As part of the set-up process, while unconnected, I have clicked “Load Settings” to get information from the local radio (the one connected to my laptop). It appears to try and connect but times out (Fails). I get a pop up window that tells me “Failed to enter command mode”. The baud rate rate and com port shown in the upper right hand side of Mission Planner are correct (I checked this using Device Manager in Windows. One thing is odd that I feel is probably connected to my problem, is that there is a third drop down box in the upper right hand corner of the MP display - see attached screenshot - that is empty. I haven’t seen any reference to this in the documentation. Any clue what is going on and how I can fix my problem?