I think it's important to mention screening.
Certified stuff may screen out ADS-B from a source with uncertified GPS, i.e. SIL = 0. Or you may be seen but not trigger TCAS. So, you won't necessarily pop up on every ADS-B in system, and the CAA agrees with this conclusion, having echoed it in their EC grant application backgrounder.
This is a point worth raising but does not alter the advice given to make the connection with the cable, set up the TT21 as per the Trig instructions, and enjoy the benefits of being seen by more aircraft.