What happens next?

How to potty train pet rats
Start by saving a tablespoon of droppings out of your rat's old cage.
Set it aside to put in the litter box, making sure the spoon carries no scent that could distract your rat from the litter box.
Clean your rat's cage thoroughly.
Put in fresh bedding and disinfect any toys or houses that might have dropping on them. Take a small, shallow box and put it in the corner where your rat generally does his duty.