As a guardsmen (TL) how do I go about having a prior army AD under my command? it seems as if the person is not taking me seriously?

I can tell you this, your leadership approach is most likely the issue... I know that sounds shitty to say, given I do not know you, but it is most likely the main issues here. You are not an NCO, you may be a TL; but you have to earn the strips and lead from the front. Look internally at your leadership style... would you take you seriously if the situation was reversed... Servant Leadership, google it, it is a great place to start for new leaders.

This is a very vague post. Also - you are not in command. Of anyone. You are someone's team leader.

Have you sat down and counseled them? If there hasn't been change and they are disrespecting you, take it up to your PSG.

It's not your place or purpose to reason with them. You were placed as Team Leader and they were placed in the team. That's the way it is until it isn't. Just do your job and ensure they do theirs. I wouldn't speak to them as a complete boot but they are in the position they are in for a reason. Don't overthink it. Prior AD doesn't add up to much if they were placed under a E4 TL.