Private chat app using wifi only. No SIM available

My niece has an old iphone with no SIM/phone number. Only wifi internet access.

Looking for an app for both of us to be able to chat

She cant chat with me over signal because no phonee number. She chats with the rest of the family over imessage, but i have an android phone

Is there an app available for both android and iphone to let me chat with her?

Thanks

Edit: willing to self-host for extra privacy. My nieces and nephews are all young children so i definitely want to encourage/educate them to get off corporate controlled communications channels