How Do I toilet train my bunnies?

My rabbits pees and poos around the entire cage. They do not have a specific place where they do their business. Wherever the litter pan is placed, they would just pee and poo all over but just not in there. How do I get them to start using it? It's a quite a problem since I'm not using a tray based cage. I do not want them to step on their piss. I have to groom them almost every day because of this.

My rabbits think that the whole house is for them to roam about hence, the entire cage is their toilet, but, strangely... they would avoid using the little pan.

Any suggestions to toilet train them?
(They live in separate cages with both with the same problem.)