A telecommunicator is located in the entrance of the Lapis Gym.
So you're going to fight Shelly, right?
By the way, did you know that some leaders in Reborn choose to fight in double battles? Shelly is one of those. Her tag team strategy focuses on setting up the rain to cover up her Fire-type weakness, and then going for a safe sweep.
If you don't let her set the rain up, Fire type attacks in her Forest Arena will make short work of her. But, if the rain does come, go with the flow and use water moves to ride that wave! Many of her bugs won't resist it, so extra power can be her own undoing.