You won't like this one - I sure didn't, but it worked.
When we moved into a new home 33 years ago, the blue tiled shower off the master bedroom was coated with soap and scum. I tried every known product available to get that scum off the tiles. Nothing worked.
Then I took a single edge razor and scraped the scum off. What a job! But it worked. The shower looked almost brand new. I did that once a year until we put in a new bathroom.
Note that I scraped the scum off over a period of weeks, and only scraped when the shower was dry.