On testosterone teplacement therapy. Understanding this makes conceiving a child hard, are there any supplements that can help?

Complex issue. The main question is why are you on testosterone. If you were started on it because your testicles are damaged, you likely would have problems with sperm production. If you are on it because of a pituitary problem, there are other hormones that can improve infertility. If you are on it for body building, you need to stop it and wait for your own body to go back to normal.