There are numerous opensource or GPL licensed mobile applications for using with XMPP servers.
The link has a list of supported clients, you can take a look at them and decide to use them or you can even build one from scratch ( 
daring move)
https://docs.ejabberd.im/developer/xmpp-clients-bots/
Checkout, the client section for the below link:
https://esl.github.io/MongooseDocs/latest/user-guide/ABCs-of-MongooseIM/
