If all you want to do is maximize range using the potentiometer is the recommended method. Simply turn it fully clock-wise and leave J1-4 floating.
J1-4 is for automated range control from a DAC (e.g. from a microcontroller). If this is what you want to do I will need more information. I don't understand what you mean exactly by "U2B", that is a part, not a net. Can you connect a ground from your power supply to J1-4? Secondly, when it doesn't work, what happens instead? Does the whole radar fail or it works but you think the range is not correct?