How to remove ring in bath tub??

See if these help you.

https://allstarsteamcleaning.com/2017/03/30/how-to-get-rid-of-that-pesky-bathtub-ring/

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

Have you tried a paste of vinegar and baking soda yet?

Try some cheap pink dish soap. I found it made old porcelain look like new. It must be the old fashioned cheap light pink stuff.

What kind of tub is it? If it is an older tub, you might try a pumie stick. They work great for hard water rings and stubborn stains on porcelain and cast iron. This method will scratch a newer fiberglass tub though.

I have applied bleach soaked cotton for a few hours on stubborn mold spots, something else to try. :)

We bought a house that had been lived in by an older gentleman who never cleaned the tub. I tried every cleaner on the market and nothing worked. Finally, I got a razor and 'shaved' the tub. It was amazing the amount of hardened soap scum that came off. After the 'shave' the kids thought we have replaced the tub with a new one it was so shiny. Hope this helps.

I tried Dawn and vinegar

Have you tried Comet? It's old fashioned...but just wet the area...sprinkle it on. It might take some elbow grease at first. But it will come clean. Don't worry....it won't hurt the tub.

I tried everything. The only thing that worked for me was "Barkeepers Friend" Still took elbow grease but the difference was amazing.