Vinegar and baking soda, make a paste. Or try Bar Keepers Friend.
We had horribly stained one piece fiberglass units. Used Easy Off Oven cleaner in the blue can. The change was amazing. Took a tub/shower unit that hadn't been cleaned in many years and made it looks almost like brand new.
I spray it on. It smells so I turn the exhaust fan on and close the door behind me. Let it sit for about 30 minutes and then using a broom scrub and rinse well.
Here's a good and safe cleaner:
Hi Tanne. My name is Cindy. I make my own cleaning solution and I love it. Very easy to make. I peel 2 oranges and put the peels in a jar. Eat the oranges. Pour vinegar into the jar to cover the peels. Put it in your refrigerator. Takes at least 2 weeks to use, but more time is better. Shake the jar every other day. Then put the liquid in a spritz bottle and throw the peels away. I use this cleaning solution in my kitchen and bathrooms and throughout my house. Smells sooo good. To clean your bath tub, sprinkle baking soda in it and then spritz with vinegar. Let it set 15 or 20 mins. Finish by wiping it all out with a clean damp cloth. Repeat if necessary. Best wishes from Cindy.