Depends on the color of the ring...orange is iron, white is calcium aka lime. if you have tried clr that’s calcium. Lime. Rust and it haven’t worked, you need a pumice stone and disposable gloves. The pumis stone won’t hurt the toilet but can be a bit awkward, to use. You need to get most of the water out of the toilet bowl. Then use the pumis stone , scrubbing the mineral deposits..
I was going to suggest pumice stone as well, they work and won't scratch, I have also used a single edged razor blade to scrape off the build up then the pumice stone, I have also used it on the drain in the sink...
The suggested Tang - koolaid internet recommendations did not work for my toilet hard water stains/ ring.
My solution was the pumice stick. This is excellent way to remove stains and rings in the toilet. When commercial toilet cleaners fail -this is my go to. Great for thick crusted calcium mineral build up that the toilet waterbowl collects.
They are found at my local Walmart for less than two dollars -good for multiple uses. Be sure to follow the use instructions, soaking the pumice stone in water for a few minutes. While the stone is soaking, grab your toilet brush and splash water up around the ring. Note: It’s very important to keep both the stone and the surface wet while cleaning to avoid scratching your porcelain.
Lime a way!! Works like a charm! Apply it, leave it on for 30 mins and it will come off so easy!