When you have burnt on grease in your oven that you need to remove you can usually get the burnt on grease out of the oven by using vinegar and baking soda.
Mix 3 tablespoons of baking soda in an empty spray bottle and then fill the rest of the spray bottle with baking soda.
Then shake the bottle to mix it up.
Next spray the oven inside with the vinegar and baking soda and allow it to sit overnight.
Then the next morning use a scrub brush or steel wool pad to clean the grease off the inside of the oven.
If that doesn't work then you may need to use some Easy Off oven cleaner which is some pretty strong and powerful stuff.
Spray the Easy Off on the burnt on grease and allow it to sit for 20 to 30 minutes and then use a steel wool pad or scrub brush to clean the burnt on grease off.