My first floor bathroom has a grayish ring in the toilet. I have tried

try coca cola. let sit before flushing

Try a pumice stone and hydrogine peroxide

Hello — After a good scrubbing you might want to put something like Rust Guard in your tank. It comes in a package of two solid tablets and there are instructions on the back . It worked for me!

Try packing on damp borax. Leave overnight. Add vinegar to tank and flush. If that does not work well enough, shut off the valve on the water supply. Flush. Pour in 2 litre bottle of Coke. Leave overnight. Flush.

I use pumice stones to clean the hard buildup in my toilets. They kind of melt as they clean and will not scratch the porcelain.

Is it possible that the ring is actually scratch marks from the scrubber? If so, you may have to drain all the water out and use a product made for restoring porcelain tubs. I think Rustoleum makes it.

Soak paper towels with vinegar and leave to soak if that doesn't work add peroxide and still leave to soak. Wipe the bowl clean with the paper towels after soaking