Here are a couple of ideas.
Try a product called "Scouring Stick" It's pumice and is available at Walmart.
The souring stick will remove it! We have hard water in New Mexico and this is the only way I’ve been able to remove build ups.
A pumice stone
Here is a good article https://www.cleanipedia.com/gb/bathroom-cleaning/clean-toilet-bowl-remove-limescale.html
good blogs post about house cleaning visit https://www.localexpertcleaning.co.uk/blogs/" rel="nofollow noopener" target="_blank">local expert cleaning
The secret to using a pumice stone is to keep it wet. Works for me.
CLR or Heat up 1 cup of white vinegar, mix in about 1/4 cup of Dawn Premium. Not the dollar store kind, because it is too thin. When mixed, pour into a spray bottle and fill with water. Shake. Spray down the grout and let it sit overnight. In the morning rinse. If you have stubborn stains, spray again. Use this mixture a few times a week. It will keep your grout and tiles clean. Also works wonders in the toilet bowl to keep hard water stains at bay. Oh you only have to heat the vinegar one time when you add the Dawn. I use this in a spray bottle and spray the toilets weekly NO MORE LIME SCALE