Try Bar Keepers Friend. Find next to the cleansers like Ajax and Comet.
Pumice Stone.. Sounds strange but it dosnt scratch the porcelain are anything! Little elbow grease and gone!! I do it all the time... good luck.
There’s many rust stain removal products and I would start out with bathtub-caliber Lyme/Calcium/Rust remover first.
Try multiple applications & consider where the rust is coming from.
If that doesn’t work, then Muriatic Acid Solution will. But it is caustic & has strong fumes and is a summertime project, because the fumes are that bad.
Use long vinyl gloves no matter what you use.
Try clr, calcium lime rust, available at Walmart and DIY stores.