Installation manual (Rev H) page 12-29 says:
"TX POWER LOW (value) = Transmit power output is low; value is Power output.
Generally, this is an installation issue with antenna, feedline, or the connector. Check the antenna, feedline, or the connector. A visual inspection is often not sufficient to find the fault with the antenna, feedline, or the connector."
So first steps I would take are to go back through all pins to make sure they are properly inserted and connectors are fully seated. Same for the feedline and then the antenna.
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.