Here’s a suggestion. I have a Trig TT-22, basically the same as the Dynon transponder. It came with a TNC (not BNC) connector. When I assembled the TNC I failed to ‘click’ or properly seat the center pin in the connector, and, although ohmmeter tests looked okay, it wouldn’t seat properly in the mating socket. Disconnect yours, see if the center pin is sticking out far enough and cannot be pushed further in.