How to get ring rid of bathtub ring ,not dirty but stained ,

https://www.hunker.com/13422322/home-remedies-to-clean-a-ring-around-the-tub

Is this an acrylic tub?

What kind of tub is it? Cast iron or fiberglass? The type of tub would dictate the recommended cleanser. Because fiberglass can be easily scratched, baking soda seems to be the less offensive of all the mild detergents listed in the resource below.

“For stubborn areas, such as bathtub rings, you can make a paste out of baking soda and a little water. Wet down the tub’s surface and gently rub the paste onto it with a sponge, soft nylon brush, or cloth.”

https://www.hometips.com/repair-fix/bathtub-fiberglass-cleaning.html

Yes, I tend to equate the two. It was important to know it is not porcelain.

I read the article recommended by Janet. Have to say that I would have been reluctant to try oven cleaner, which is a strong alkali. On the market there is also an acid cleaner and it is recommended for fiberglass fixtures by the seller.

You'll want to avoid abrasives like cleansers unless the cleanser manufacturer specifically recommends using it on plastics. The Mr. Clean Magic Eraser is recommended by many on this site and is not abrasive.

I would be interested in knowing what has not worked and when you do get it clean what did work. Hope that you will let us know.