They say Biotin helps. You can read up on different doses that are recommended.
I also have hear good iron levels help, but iron you really should check by a blood test before taking a lot of it. You would want to find out if you are actually deficient or not. With hair that long I doubt you have a deficiency.
I think hair length is fairly programmed. Just like your eyelashes grow to a certain length I think hair on your head does too. I don’t know the actual science on that, but it makes sense to me.
FYI: most people on TV with full long hair have extensions. It’s not all their hair.