To attempt to fix the cracked or broken plastic knob yourself, you can use epoxy glue. Apply the epoxy to the cracked area of the knob and hold it together while drying with a pair of needle nose pliers. Hold the plastic together for the amount of time the epoxy instructions advise you.
How do I fix my oven dial?
https://www.youtube.com/watch?v=mt3nNFEAPPs
How do I fix my GE dryer knob?
https://www.youtube.com/watch?v=02EG27Jgoi0
Can you fix a broken start knob on a dryer?
To attempt to fix the cracked or broken plastic knob yourself, you can use epoxy glue. Apply the epoxy to the cracked area of the knob and hold it together while drying with a pair of needle nose pliers. Once the glue is dry, slide the dryer knob back onto the timer shaft.