I've tried everything even the Works
Sand it down and paint
Try a product called Scouring Stick. you can get it at Walmart.
We've used Iron Out with lots of success.
Try Coke: https://www.craftylittlegnome.com/clean-toilet-coke/
Lysol Lime & Rust Remover Toilet Bowl cleaner, comes in a black squirt bottle. Scrub with Scrub Daddy or Mr. Clean Magic Eraser. Emery scrubbing block or Scouring Stick may be needed for super stubborn stains.