How to clean convection microwave ceramic turntable?

You can try putting a microwave safe bowl or cup of water in and let it boil so the steam gets all over the inside then wipe out. Might take more than once if it is really burned on. If you need scrubbing power try Barkeepers Friend powder with a Magic Eraser scrub pad.

The turntable should be able to be cleaned with oven cleaner.

Is the plate removable?I would not use any cleaners except natural like baking soda and vinegar

Just pour some vinegar on the plate and leave it in the closed microwave for an hour. Then, rinse. Scrub the plate with baking soda and water (or another foodsafe kitchen cleaner), or with salt and a scrubbing sponge. Thoroughly rinse and dry with a cloth. If the burned food area covers the sides or bottom of the plate, you might prefer to soak the plate in warm water, vinegar, and baking soda in a sink or large pan, then scrub. Best wishes ☺️

Soak it with dish soap and a dryer sheet

Really, look up the maker of the oven, call them and they will tell you everything. Do it all the time.

Thank you everyone for you helpful suggestions. Very much appreciated