What should I do about cracks in the cement pan for a roll-in shower?
Unfortunately, your real problem here actually isn't that you have cracks in the cement; it's that your cement has developed several cracks in 2 days. They are a symptom indicating that the cement is weak (typically because it was not mixed correctly; most often, it's too wet). Nothing you do to repair the cracks will fix that weakness.

The right thing to do in this case would be to take the cement out and start over.