Gentle products will help with small deposits - soap, dishwashing gel, washing powder solution.
To remove local contaminants, use alcohol or table vinegar.
Heavy stains can be removed with industrial bleaches or specialized sprays.
A soap solution that can be made with grated laundry soap and water works great with yellowed plastic - mix the ingredients until it becomes a paste, wait until it swells, apply to the surface for half an hour and rinse with warm water.
There are many options for using hydrogen peroxide in the home, it is not only a disinfectant.
It will also help deal with yellowed plastic. Proceed by analogy with acetone - apply to a sponge and wipe the dirty area.
For a more obvious effect, you can repeat the procedure several times in one day.
Since it is a little more difficult to bleach plastic with vinegar, for more stubborn stains you will need 80% essence.
Be sure to wear gloves and a mask, then soak a sponge in vinegar and scrub away any stains.
Dimexide is very popular - a solution that can be found in any pharmacy.