Need advice on Registration and Health Insurance (Frankfurt)

I'm in a unique situation. Im a German national but born and raised in Asia. Ive lived there my entire life. I recently left my full time job and entire life to move to Frankfurt to be closer to my dad who is unwell and undergoing treatment. Cant find work as I don't speak German. I live in my dads apartment for 3 weeks now and need to register myself as living in Frankfurt but I am not on the lease agreement, was told that I need documents to prove I am living in the apartment before I can get myself registered? Not sure how to go about this.... In addition am not sure how I can even get the mandatory German Health Insurance as I have not worked here before and Im unemployed currently.