I’m not sure but I seem to remember that the computer ethernet BNC connectors were 75 Ohm not 50 Ohm? You should be able to see by looking at the size of the hole for the centre conductor, 75 Ohm ones are larger.
You have an Auto button and a Tune button. I thought the unit was auto (i.e. RF triggered) by default. Does the auto button turn than on and off so that the Tune button has to be pressed?
73 Ed.