Once transmitter and receiver are powered on and bound properly you should see the rx battery bar graph. There are 2 settings in the menu. One for number of lipo cells (mine on 12s) and one for nominal voltage per cell (3.7 for lipo).
I think the battery ground needs to be connected to the rx input ground wire to work. In other words they need to share the same ground.
Verify that you are getting full battery voltage to the location where you soldered to the 42v input.